THALLOPHYTA. 627 



the surface layers of the sea; some 30 species have been found amongst the inor- 

 ganic dust of the snow-field. There are some 2000 species of living forms. 



Considerable deposits of Diatoms occur in various parts of the world; of these 

 the most remarkable is that of Richmond, Virginia, U.S.A. It is said to extend for 

 many miles and to be 40 ft. deep. They are found in secondary, tertiary, and more 

 recent rocks. Siliceous marl consists entirely of the tests of Diatoms. A block 

 of such a Diatom-deposit some two cubic feet in bulk from a fresh-water lake in 

 Australia is exhibited in the Botanical Department of the British Museum; the 

 number of Diatoms contained in it (there are 21 different species) is estimated as 

 exceeding 12 billions (12 x 10^-). 



Class IV.— GAMOPHYCE^. 

 Sub-class I. — Chlorophyce^, the Green Algoe. 



There can hardly be a more fascinating group of plants than this, whether to 

 the strictly scientific botanist or to the more catholic lover of nature. In the first 

 place, the Green Algae are among the most widely diffused of plant-forms. They 

 grow practically in every place where enough moisture, together with light and air, 

 are to be had. Between tide-marks on almost every coast, floating on the surface of 

 the deep sea, covering damp earth, walls, palings, and tree trunks, sticking to the 

 surface of leaves in the moist atmosphere of tropical forests and jungles, and 

 inhabiting almost every river, brook, pond, ditch, or casual pool of rain-water in all 

 quarters of the globe, are members of this ubiquitous group to be found. Nor are 

 they wanting from more extraordinary situations. In Switzerland, Norway, and 

 other countries where snow is more or less permanent, the bright red patches on its 

 surface, known as "red snow", are formed by the microscopic Alga {Sphcerella niva- 

 lis) shown in fig. 25a, of the first volume. Other forms, not so far removed from 

 Sphcerella, live in the intercellular spaces of higher plants, such as the Ivy-leaved 

 j Duckweed {Lemna trisulca), the Moneywort (Lysimachia nuinvmularia), and 

 others. Yet other Algae are found inhabiting the jelly of certain fresh-water 

 sponges in the East Indies, where they seem to live in a regular symbiosis with 

 their hosts. But perhaps the most curious dwelling-places of all are the hollow 

 hairs of the Three-toed Sloth {Bradypus tridactylus), which are inhabited by an 

 Alga called Trichophilus. 



In the second place, many of the Green Algae (and among these some of the 

 very commonest) are the most beautiful forms of life imaginable, and the main 

 features of their structure can be made out with quite low powers of the microscope. 



But perhaps the greatest claim on our interest is made by the fact that we must 

 look among the Green Algae not only for indications of the origin of all plant life, 

 and of the forms from which the whole of the higher plant world arose, but also for 

 hints towards the solution of some of the most difficult and fundamental problems 

 with which Biologists are occupied, quebtions as to the real nature and origin of 

 sexual reproduction, as to the distinction between gametes and asexual reproduc- 



