Minute marvels of nature : being some revelations of the microscope

Front matter

Transverse section of young Beech stem, showing internal structure

Actual diameter i of an inch uo

BEING SOME REVELATIONS OF THE MICROSCOPE

Chapter 1 The Beginnings Of Plant Like

Microscopic plants on an old wooden fence -Only a small proportion of the world’s plants assume conspicuous forms — Sea-water coloured with minute plants Difficult to find any natural condition of land or water free from plant life — Plants which swim freely about in water — Difficulties in distinguishing between the lower plants and animalcules — Desmids — Structure of ditto — Reproduction of ditto — Diatoms — Where found — Structure of ditto — The decorative markings and sculpturings of ditto — “ Rotten-stone” or “Tripoli" — Diatom deposits eventually form rocks — Varieties of forms assumed — Movements of diatoms — Revealing power of microscope — Man’s skill with the minute — Thread-like plants — Plants which show the first indications of a stem, and differentiation in its cell-structure — Gradual evolution from lower to higher life-forms Pp. 1-31

Chapter II

Green film encrusting old fence consists of many hundreds of plants — Structure of these unicellular plant-atoms — Reproduction of ditto — Red snow — Vegetable cells are the bricks which build up the plant edifice — Hairs on plants — The

XIV

CONTENTS velvety appearance of flowers’ petals, how it is produced— Vascular tissues — Monocotyledons — Dicotyledons — Tissue which forms new cells — The rough bark of trees— The age of trees, how it can be estimated — Varieties of stem structure— Stems and leaf-stalks — Structural botany uninteresting

Chapter III

All life depends upon the activity of a certain green-coloured substance which is found in the tissues of leaves — The structure of a laurel leaf — The growing-point of a stem — Variations in leaf structure — Chemical analysis of a green leaf — Great trees chiefly built up of carbon obtained from the atmosphere by green leaves — “Fall” of leaves — Plants and animals dependent on each other — How leaves purify the air — Chlorophyll corpuscles — Plants not only supply us with oxygen, but all our food and innumerable home comforts— Canadian woodland and forest — Chlorophyll the mainstay of life — Enormous quantities of carbon dioxide passed into the atmosphere daily . , . Pp. 58-83

Chapter IV Pollen, Ok Flower-dust

Sex exists as much amongst plants as animals — Male and female flowers of a begonia — Functions of coloured portions of flowers — Stamens from various flowers — Fertilisation of foxglove — Microscopic examination of pollen — Structure of the pollen-grain — Fertilisation — Various kinds of pollen — Enormous quantity of pollen produced — Showers of pollen falling in the streets of towns — Marvels of function carried on beyond the range of unaided human vision . Pp. 84-107

CONTENTS

XV

Chapter V

Sea-weeds have no strong branches to support their weight — Sea-weeds that are not sea-weeds — Animal-plants — I he structure of ditto — Fossil species — Mediterranean corals — The polypes — Their resemblance to flowers and seeds — The hard red coral of necklaces, &c. — Polypes the builders and not man- Alga;, or sea weeds proper — Their structure Alga; with hard and stony fronds — Lime-builders

Pp. lofl 1 31

Chapter VI

Eggs of insects present fascinating objects for microscopic investigation Maggots no longer supposed to be spontaneously generated from putrefying substances Eggs of the common house-fly — Number of eggs deposited varies — Successive generations and production of living young- -Aphides or “green flies" — The hover-flies — The lacewing-fly — The wonderful eggs of birds’ parasites — Structure of insects’ eggs — Eggs of butterflies and moths — Where found — Instinct of the mother insect — Blunders made by insects in depositing their eggs — Varieties of form— -What purpose the artistic and microscopic sculpturings may serve . . . Pp. 132-156

Chapter VII

All living animals pestered more or less by parasites adapted to prey upon them — Insects no more exempt than larger animals— Hyper-parasitism — Parasites which require several

XVI

CONTENTS hosts to complete their life-cycle — The sheep-tick — Various animal parasites, beneficial and otherwise — Study of parasites offers a prodigious field for scientific work . Pp. 157-183

Chapter VIII

Many insects which can inflict serious injury with their weapons — The gad-fly — Personal experience of the effect of its weapons — The malarial mosquito — The weapons of the wasp — The spider’s poison fangs and teeth — A fly’s foot — The foreleg of a diving beetle — Feathered oar of a waterboatman — Poison bag and sting of wasp — An insect armed with a bayonet— -The saw-fly’s weapons — Every insect has some special weapons to suit its own particular purposes

Chapter IX

Insect life at the bottom of the pond — Aquatic insects spend the greater part of their lives beneath the water — The May fly as the type of brief and ineffectual life — Their anatomy — Their graceful and buoyant movements — Life story — Extraordinary final emergence — The alder-fly — Caddis-flies — 1 he wonderful cases built by the caddis-grubs — Dragon-flies — Mask of the dragon fly larva — Strength of wings — Eating butterfly fp- 207-233

Chapter X

The mouth of a snail is provided with teeth unlimited — Genera and species identified by the various characteristic dental

CONTENTS xvii

arrangements — Slugs which eat worms — Tongue or proboscis of the common blow-fly— Proboscis of hover-fly — Tongues of butterflies- Sap-sweeping tongue of the largest British beetle Pp. 234-243

Chapter XI Simple Wonders Of The Microscope

Dust rubbed from the wings of a moth Scales of moths and butterflies — Hairs of insects -Skins and scales of fishes — Feathers of birds — Varieties in birds' feathers, and why they vary -Hairs of mammals Hairs are modified skin structures Human hair Hair sections — Conclusion . Pp. 244 ’64

Index Pp. 265 272 b

LIST OF ILLUSTRATIONS

Transverse section of beech twig . . . /■><«•/..//<. t

1. Unicellular plants multiplying 3

2. Desmids (Closterium lunula) ...... 5

3. Desmids ( MicrasUrias denticulata) 7

4. Diatoms grouped 10

5. Diatoms on a dark ground 12

6. Diatoms dredged from the Atlantic .... 14

7. Diatoms common in ponds and lake ' . . . . 15

tS. Diatom-chains 16

g. Selected diatom Arachnoidisats Ehrtnlurgii\ 17

10. Central portion of diatom group. Fig. 4 . . . 18

11. Selected diatoms from group, Fig. 10 . . . . ig

12 Selected diatoms from group. Fig. 4 .... 20

13. Artistic arrangement of diatoms 21

14. Central portion of diatom group. F'ig. 13 . . . 23

15. Another artistic arrangement of diatoms ... 24

16. Central portion of group. F'ig. 15 26

17. Thread-like plants ( Spirogyra ) ..... 28

1 8. Volvo. v globator 29

19. Freshwater alga ( Draparnaldia ) 30

20. Freshwater alga (Batracliospermuni) .... 31

21. Alga on damp fence ....... 33

22. Vegetable cells of apple 33

23. Flairs from flower of pansy ...... 37

24. Cells from surface of geranium petal .... 38

25. Scales from fern frond ....... 3^

26. Vascular tubes in stem of bracken fern ... 40

XX LIST OF ILLUSTRATIONS

H1G- I'AGK

27. Section of sarsaparilla stem 42

28. Section of sycamore stem ...... 43

29. Section of clematis stem 46

30. Section of date-palm stem ..... 47

31. Section of pilhvort stem 48

32. Section of marestail stem 49

33. Section of club-moss stem 50

34. Section of rush stem . 51

35. Section of ivy stem . 52

36. Section of Brazilian liana stem 54

37. Section of mid-rib of rhododendron leaf . . 55

38. Section of pine leaf 56

3g. Section of blade of laurel leaf 60

40. Section of mid-rib of laurel leaf 61

41. Section of blade of sunflower leaf 62

42. Section of mid-rib of sunflower leaf .... 63

43. The growing-point of a stem 65

44. Section of blade of deadly nightshade leaf ... 66

45. Section of stonecrop leaf 67

46. Section of pine leaf. 69

47. Section of blade of water-lily leaf 70

48. Section of blade of Indian corn leaf .... 71

49. Transparent leaves of a moss 73

50. Section of the base of a Virginian creeper's leaf stalk . 77

51. Breathing pores or “ stomata ” of monkey-puzzle leaf . 79

52. “ Stomata ” of tulip leaf 81

53. Central portion of male begonia flower .... 85

54. Central portion of female begonia flower . . . 86

55. Stamens from various flowers 88

56. The tubular corolla of foxglove opened ... 90

57. Pollen grains falling from stamens of mallow . . 92

58. Spiny pollen grains of hollyhock 93

59. Ripe stamens of mallow flower ..... 94

60. Section of anther of a lily ...... .95

61. Compound pollen grains . ...... 96

62. Sections of pollen grains ...... 98

63. Stigma of evening primrose with pollen tubes . . 100

Kit;.

LIST OF ILLUSTRATIONS

XXI

64. Pollen from evening primrose ....

65. Triangular pollen grains

66. Reticulated pollen grains ....

67. Pollen of Monarda

68. Pollen of vegetable marrow ....

69. Pollen of pine ....

70. Animal-plants

71. Branches of an animal-plant

72. A coralline animal-plant

7j. Animal plant cells arranged in a leaf-like manner

74. Fossil zoophytes

75. Section of limestone ......

yb. Animal-plant feeding

77. Section of fossil corals .....

78. Branches of a tiny sea-weed

79. Sea-weed scattering its spores ....

80. Tip of frond of delicate sea-weed

81. Sea-weed discharging its capsule of spores .

82. Sea-weed with stony foliage ..... S3. Another sea-weed with stony branches

84. Stony sea weed with its companion stone-masons

85. Kggs of the common house-fly ....

'86. An egg of a hover-fly

87. Eggs of parasite of the ground hornbill

88. Eggs of parasite of Australian mallee-hird .

S9. Eggs of parasite of turkey of Japan

90. Eggs of fowl parasite

9 1 . Eggs of pheasant parasite .....

92. Eggs of peacock parasite

93. Eggs of moth on cabbage leaf

94. Eggs of Privet Hawk-Moth (natural size) .

95. Eggs of Privet Hawk-Moth (magnified)

96. Eggs of Currant Moth

97. Eggs of Blue Underwing Moth ....

98. Eggs of Gray Chi Moth

99. Eggs of Small Emerald Moth ....

100. Egg of the Brown Hair-streak Butterfly iAi.n

>°5

1 10

1 1 1

1 12

”4 1 16

1-24

>25

>3°

>34

>37

>39

142 >43 >44 >45 146

>47

>49

>52

XXII

LIST OK ILLUSTRATIONS

I'AGE

FIG.

101. Eggs of the Small Tortoiseshell Butterfly . . . 153

102. Eggs of the Common Meadow-brown Butterfly . . 154

103. Eggs of the Small Copper Butterfly . . . .155

104. Parasite of tortoise 139

105. Parasite of humble-bee 160

106. Parasite of humble-bee, fore-parts .... 161

107. Beetle and parasite 164

10S. Pineer of beetle parasite 165

109. Parasites of house-fly ....... 166

no. A sheep tick 169

hi. Foreparts of sheep tick i?°

1 13. Parasite of ostrich 174

114. Parasite of crow ... .... 175

1 15. Parasite of pigeon 176

116. Parasite of tawny owl 17®

1 1 7. Parasite of pike . . . . . • . *79

1 18. Parasite of polecat ....... 181

1 19. Parasite of bat 182

120. Lancets and blood-suckers of gad-fly . . . 185

121. Tip of gad-fly’s lancets 186

122. Tip of gad-fly’s blood-sucker . 187

123. Another view of the gad-fly’s mouth .... 188

124. Mouth-weapons of the female mosquito . . . 19°

125. Head of male mosquito !9X

126. Mouth of wasp 192

127. Mouth of spider i93

128. Foot of fly 195

129. Foreleg of a water-beetle 19b

130. Feathered oar of water-boatman ..... J97

131. Poison-bag and sting of wasp *98

1 32. Egg-depositor of an ichneumon fly .... *99

133. Another insect bayonet ...... 21:0

134. Ichneumon fly’s foot ....... 201

135. Ichneumon fly which lays its eggs in “ blight . . 202

13b. Tip of saw of saw-fly 2l>3

137. The saw-fly’s weapons 2<>4

LIST OF ILLUSTRATIONS

.win

138. The caterpillar’s many-hooked foot .... 206

139. Portion of wing of May-fly 210

140. Larva of May-fly 212

141. Exuvium of May-fly 214

142. Final emergence of May-flv 2if>

143. May-fly (natural size)

144. An alder-fly • .219

145. Eggs of alder-fly 220

146. Larva of alder-fly 221

147. A caddis-flv 222

148. A caddis-worm 223

149. Specimens of caddis-cases (natural si/e) . . . 224

150. End of body of caddis-larva 22b

1 51. A dragon-fly (natural size) 227

152. Larva of dragon fly ....... 228

153. Head of the larva of a dragon-fly .... 230

154. Tips of wings of dragon-fly 231

155. The face of a dragon-fly 232

156. Palate of edible snail 235

157. Teeth of edible snail 23b

158. Palate of snail showing different forms of teeth . . 237

1 59. Palate of sing 238

160. Proboscis of blow-flv ....... 239

161. Proboscis of hover-fly . 240

162. Tongue of a butterfly 241

163. Tongue of stag-beetle 242

164. Scales or dust from wing of moth .... 245

ib5* Scales on butterfly’s wing, in situ .... 24b

166. Hairs of “woolly-bear ” caterpillar .... 247

167. Hairs of a humble-bee 248

ibS. Leg of a tiger-moth 249

170. Skin of sole 251

171. Skin of eel 252

172. Scale removed from eel-skin 253

173. Scale removed from gold-fish ..... 254

174. Feather of humming-bird 255

XXIV

LIST OF ILLUSTRATIONS

I'AGE

KIG.

175. Feather of condor . .....

. 256

176. Feather of ostrich .....

• 257

177. Feather of emu

. . 258

178. Feather of owl

• ■ 258

179. Section of mouse-skin .....

• 259

180. Mouse hair

181. Shavings from human beard .

182. Sections of whisker-hairs of lioness

183. Sections of hairs of American peccary

• • . 263

1S4. Sections of tail-hairs of African elephant .

• 263

Chapter I

At the end of my garden, facing full south, stands an old wooden fence. Nothing could appear more thoroughly and completely dead than a paling which is beginning to decay ; but ii you will come with me to the fence I will show you more living plants than you could observe in a bird’s-eye view of the whole of Kew Gardens. Many of us think of “ plants ” only as the flowering plants which are put in our garden, and we should see no absurdity in remarking that a flower-bed contained “ more weeds than plants ; ” while very few would enumerate more than trees, shrubs, herbs, grasses, ferns, and mosses as classes of plants. Yet only a very small proportion of the world’s plant life, so far as numbers go, assumes these prominent forms.

The sea, for instance, is sometimes conspicuously tinged in large patches, upon which inexperienced passengers gaze in wonder from the steamer’s deck, by plants. You may fill a tumbler with the

A o coloured sea- water, and no matter how closely you look, it still seems only coloured water. But the colour is due to incalculable multitudes of tiny plants, living their separate sea-lives as completely as the great whale himself. In the same way a coloured stain will creep over damp walls, the bark of trees, or this old fence of mine, v\ here myriads of microscopic plants are congregating together and multiplying very rapidly.

For the infinite variety of form and habit that plant life assumes adapts it to flourish in sites where life of any kind might have seemed impossible. Leathery and powdery plant-incrustations cling to the hardest rocks and stoniest soils ; vegetable moulds take up their abode on almost every perishable article ; trees and ships are often completely destroyed by a plant multitude known as “dry-rot ”; while smut, rust, bunt, and other familiar forms of parasitic fungi, prey upon living plants to such an extent as to spoil and destroy whole crops of grain or fruit. What seems worse still is that many of them not infrequently invade the organs of living animals, and are the known causes of many diseases. A single drop of pond, river, or sea water will often reveal multitudes of varied plant forms ; and, in short, it is difficult to point to any natural condition of land or water that is free from plant life. Its germs, or spores, fill the air we breathe and are consumed in all the food that we eat. The very ground we stand upon may be built up of tiny plants, as 1 will show later.

While mentioning fungi, moulds, and other

Fig. 1. Microscopic unicellular plants, multipl)ing by division of tin ir cells parasitic organisms as being representative of plant life, it is not my purpose here to consider this class of plants, but rather those which may be regarded as leading the way to the higher plants, by their possession of the important green colouring-matter known as “ chlorophyll,” or its equivalent; of which I will speak in a later chapter.

Almost all damp situations and standing waters, such as rain-water cisterns, drinking-troughs, wet ditches, ponds, &c., will provide examples of minute algae, or the earlier forms of plant life. And these, like the green film on the fence, will be mostly unicellular plants — each microscopic cell constituting an individual plant, which eventually divides into two or four similar cells, with the same power of division (see Fig. i).

Sometimes the newly formed cells have long cilia, with which they swim freely about in water ; and stagnant ponds often owe their green hue to myriads of these active green cells swimming gaily about within them. When shown under the microscope, perhaps the last thought that would occur to the inexperienced observer would be that these wonderful little organisms are plants at all ; and as there are large numbers of lowly plant forms that can move about in water, and are almost invariably found in company with minute animalcules similarly endowed, even experts are often puzzled to decide to which kingdom the tiny living forms belong. So it comes about that many of these organisms have been bandied about by learned scientists from the Animal to the Vegetable Kingdom and back again, until the ordinary student scarcely knows where to locate them.

In recent years, however, a better understanding' has been arrived at with regard to the more

Fig. 2. Desniids ( Closterium lunula ). or unicellular plants, found in standing waters. Actual length about T,V, of an inch familiar forms, since a large number of these gaily swimming organisms have been shown conclusively to belong to the Vegetable Kingdom.

Fig-. 2 shows a number of the singular unicellular plants which are called “desmids.” These are minute fresh-water algae, of a beautiful green colour and numerous diverse forms.

Every moderately clear pool or ditch will provide examples of these interesting plants, and they especially abound in ponds which lie in exposed and bleak situations. The species in the illustration is characterised by its crescent-like form. Each cell is free and able to move about in the water by a curious, feeble movement, and if kept in a glass vessel they all move slowly to the side next to the light and congregate there. For a period of over twelve months I kept a large quantity of these desmids propagating in a common glass jar, having accidentally gathered a few attached to some of the common duckweed which floats on ponds.

Desmids, like other green plants, evolve oxygen in sunlight ; and so, together with the duckweed, which also multiplied in the glass jar, the water was kept fairly pure, and the desmids increased at such a rate as to completely line the sides of the glass vessel with a film of bright green colour.

A desmid cell possesses a thin outer coating or cell-wall, sometimes adorned with spiny projections or markings. Surrounding this, a transparent film of gelatinous matter is recognisable, although sometimes only by its preventing the cell-wall from touching external objects. Inside the cell- wall proper is a layer of colourless protoplasm, which encloses the mass of green -coloured living substance of the cell or plant. As these plant cells all show more or less clearly a line of division in the middle, they might seem to he two separate cells joined together ; hut such is not the case, each desmid being but one cell. When reproduction is about to take place, the

A ^

Fig. 3. Other forms of desmids, chiefly Micrasterias denticulata. Actual length about tJ,jj of an inch

contents of the cell divide into the two halves, which seem to draw away from each other, and so become constricted towards the centre, and finally break away. Each irregular half then soon develops the symmetrical crescent shape. The above example (Fig. 3) is taken from other species of desmids which present a somewhat different method of reproduction. Instead of breaking away and subsequently acquiring a complete shape, the two halves remain together, but gradually develop at their line of junction, two little rounded discs touching each other at the edge, which gradually become larger and push each other further apart, until, finally, the indentations round the desmids edges appear, after which the two halves become detached as perfect desmids.

There are other and more complicated methods of reproduction and cell division even with this same Micrasterias ; but these need not concern us here, though they show that even these lowly, singlecelled plants gradually approximate in structure and methods of reproduction to higher and more complicated forms. And it should also be observed with regard to these delicate and beautiful organisms, how frequently symmetry and regularity of form prevail. Rut this we will consider more in detail hereafter.

Meanwhile let us glance at some of the most extraordinary forms of the invisible vegetable world — tiny unicellular plants of almost unimaginable minuteness, called “ diatoms.”

These microscopic and wonderful algte are abundantly distributed in nature. \\ herever exposed water is to be found, diatoms may be looked for, whether it be stagnant or running, salt or fresh, warm or cold. Even the melting snow on the summits of the highest mountains, or the water that lodges in your rain-water spout, frequently contains diatoms in abundance.

As the water evaporates, these invisible plant particles become dry, and by reason of their lightness and tenuity get wafted, still alive, from one region to another by the high winds. The hottest sun and bitterest cold does not affect their vitality. As the air calms they settle down again, and after months of frost or scorching sun, given moisture and sunlight, they again rapidly multiply, and so become distributed everywhere.

When considering the desmids 1 mentioned that the cell-walls sometimes developed spines or markings on their surface. Now diatoms present most extraordinary and remarkable features in this respect. Each little vegetable cell that constitutes a diatom has the power to absorb irom the surrounding water that chemical combination termed “ silica or dint, a small proportion of which exists dissolved in most natural waters. The silica which it thus appropriates becomes deposited regularly on the cell-wall, and so the single vegetable cell becomes clothed with an almost indestructible flinty armour. But this is not all ; one side of the shell or frustule becomes slightly larger than the other, and fits over the smaller, after the manner of the halves of a pillbox or canister.

On the surfaces of these flinty shields delicately carved and chased symmetrical markings

Fig. 4. An arranged group of the cleaned shells, or shields, of the tiny plants called “ diatoms." '1 he actual group measures ,*5 of an inch in diameter are revealed when considerably magnified. An arranged group of these tiny and wonderful shields is shown in Fig. 4.

As this group measures only one-twelfth ol an inch in diameter, it will be understood that the shields are considerably magnified, although but

THE BEGINNINGS OF PLANT LIFE n the faintest indications of their fascinating sculpturings are visible.

The refined nature of some ot the markings ol diatom valves defied the powers of the best microscopes for many years, and even with the finest of modern lenses most skilful manipulation is required to bring out their details of structure. In fact, the more delicate forms are used by the makers of microscopes for testing the accuracy of their work.

A very remarkable fact regarding these minute sculpturings is that frequently, as the magnifying power is increased, new and unsuspected details of structure become visible. What were minute rows of dot-like spaces reveal, when enormously magnified, other perforations and sculpturings within them of equal beauty and symmetry.

And, when we have exhausted the powers of our best optical instruments, who knows what lies beyond? If unsuspected marvels of fascinating minuteness have been revealed with the advance of modern instruments, one cannot help wondering where this decoration ends. The methods by which these tiny plants construct their almost indestructible shells defies all attempts to explain ; and in saying “almost indestructible” I am well within the mark, for these silicious frustules, or valves, resist putrefaction almost indefinitely ; and as the organisms multiply very rapidly by subdivision, and keep falling in a steady shower to the bottom of the water as they perish, layers of these

Fig. 5. Some diatoms (Pinnularia nobilis) shown on a dark ground to illustrate their glassy nature. Individuals vary in length from ^ to VJ5 of an inch glass-like shells gradually result in considerable deposits of solid flinty material which resists acids and even the dull red heat of an ordinary fire.

The familiar polishing material “ rotten-stone ” or “ Tripoli,” large deposits of which occur in Bohemia and other parts of the world, is a soft friable rock which chemically is almost pure silica; but the microscope reveals the fact that it is not ordinary silica, but is built up of the tiny and beautiful silicious shields of diatoms. And as the forms found in rotten-stone are characteristic of fresh-water species, we conclude that where the deposits are found lakes or marshes existed in previous ages.

In long periods of time, by the slow percolation of water, these diatomaceous deposits (a minute speck of a diatom deposit composed almost entirely of one species is shown in Fig. 5) are slowly dissolved and then redeposited as a hard opal-like rock. Thus these apparently insignificant plant atoms, by their vast numbers and rapidity of multiplication, play most important parts in the formation of the earth. The city of Richmond, in Virginia, is said to be built on a stratum of diatoms 18 feet thick. Estuaries and harbours are frequently considerably shallowed by the accumulation of these flinty deposits, and the surface and ooze of many seas reveal diatoms in abundance.

Fig. 6 shows diatoms dredged from the Atlantic by the Challenger at a depth of 1990 fathoms, or just over 2^ miles.

The variety of forms assumed by the frustules, or shields, are as varied as their surface markings, and quite beyond description. The most familiar species found in ponds assume the long oval or boat-shaped forms, such as those shown in i4 MINUTE MARVELS OF NATURE

Figs. 5 and 7, and many are extremely delicate ; but all alike possess their characteristic wrink-

Fig. 6. Diatoms dredged from the Atlantic at a depth of about 2\ miles lings, clottings, and markings, when highly magnified.

It is extremely interesting to watch the peculiar gliding movements of some of these boat-shaped atoms. Without any visible effort or means of propulsion they slowly glide in a straight line through the water, and should anything obstruct

Fig. 7. Diatoms common in ponds and lakes their passage they make no effort to round it, but without turning reverse their motion and glide backwards, apparently on the same straight track on which they came, often repeating the movement several times as if unconvinced that the path was impassable.

Other common forms do not lead such a free

Fig. 8. Chains of diatoms in their natural state adhering to a larger microscopic alga, magnified

life, but remain attached, and form chains of somewhat oblong or square frustules, each adhering to the other at the corners, the chains ultimately being fastened to, or intermixed with, other and larger algae or aquatic weeds. F ig. 8 shows an example of diatom chains in their natural situa-

Fig q. An individual diatom-shield [Arachnoidiscus Ehrenbergii) immensely magnified tions, clinging to a much larger alga, although the portions shown of this are really microscopic. Others develop foot-stalks, by means of which they attach themselves to stones, &c., and so form little groups or clusters.

I he beauty of the marking's of the frustules cannot be readily seen in the living diatoms ; but the skeleton shields, when cleansed of their living matter by heating to red heat or dissolving by

Fig. io. The central portion of the group of silicious diatom shields shown at Fig. 4, more highly magnified.

nitric acid and magnified, reveal to perfection their wonders of symmetrical precision and minuteness. One of the cleaned shields from a marine form is exhibited in Fig. 9. Two hundred of these, placed edge to edge, would scarcely extend one inch ; yet this is a very large form, comparatively speaking. The lines and markings on some species are estimated at about 1 -76,000th of an inch

'Fig. ii. A few selected diatom -shields from the central portion of Fig. io again shown under increased magnifying power apart, so that a hair from the human-head cut longitudinally into 400 thin slices would approximately fill the spaces between 400 of these marks.

We cannot here even glance at the life history

Fig. 12. Several diatom forms from the top central edge of Fig» 4 immensely magnified

and methods of reproduction of these lowly and beautiful unicellular plants. The subject is so great and their species are so numerous that we should require a separate chapter; so 1 must be content to call attention to their beauties, which show that the lowest and most minute forms ol plant life present no less wonderful intricacies ol structural detail than the highest and largest.