January 16, 1903. J 



SCIENCE. 



97 



the}' live, conditions which are never met 

 with in the life of most marine organisms. 

 This necessitates some means of surviving 

 the periods of drought, and has resulted 

 in the development of various devices for 

 carrying the plants through from one 

 growing period to another. While a few 

 low aquatics, like Pleurococcus or Oscilla- 

 toria, may become completely dried up 

 without being killed, in most fresh-water 

 algffi there are produced special cells — 

 spores — which are more resistant than the 

 vegetative cells and survive the death of 

 the rest of the plant body. These resting 

 spores may be produced non-sexually, as 

 in Nostoc, or the ' aplanospores ' of some 

 of the green alg£e ; but more commonly they 

 are the product of the union of sexual 

 cells, or gametes, and may be generally 

 denominated 'zygotes.' 



This condition of things of course pre- 

 cludes growth, except when an abundant 

 water supply is provided. It is evident 

 that any device by Avhich the vegetative 

 life of the plant can be prolonged is an 

 obvious advantage. 



Some such contrivances, of a simpler 

 kind, are seen in some of the lower green 

 plants. Thus the gelatinous mass in which 

 the filaments of a Nostoc colony are im- 

 bedded, or the ' palmella-stage ' of some 

 Confervoidere, offer a certain amount of 

 resistance to the loss of water, and serve to 

 prolong the period of vegetation. Less 

 commonly root-like organs are developed 

 which enable the alga to live on the wet 

 sand, penetrating into it and drawing up 

 water from below. Species of Vaucheria 

 and Botrydium exhibit this very well. 



We may imagine that some algal form, 

 perhaps related to the existing Confervoi- 

 dese, adopted a similar amphibious habit, 

 developing rhizoids, by means of which it 

 could vegetate in the mud after the sub- 

 sidence of the water in which it was grow- 

 ing, in a manner analogous to that exhib- 



ited by cei'tain amphibious liverworts still 

 existing. The well-known Ricciocariyus 

 natans, for example, lives first as a floating 

 aquatic, but may later settle in the mud, 

 as the water subsides, and there vegetates 

 much more luxuriantly than in its aquatic 

 condition. 



The change from a dense medium like 

 water to the much rarer atmosphere neces- 

 sitates the development of mechanical tis- 

 sues, to give the plant the requisite support 

 in the air. There must also be developed 

 devices for protecting the tissues against 

 excessive loss of water due to transpira- 

 tion. Other modifications are to insure 

 economy of water in fertilization. 



In submerged aquatic plants water is 

 absorbed directly by all the superficial 

 cells, and of course there is no loss due to 

 transpiration. Moreover, special conduct- 

 ing tissues are made less important, and 

 are either quite wanting, as in most alg£e, 

 or much less developed than in related 

 terrestrial forms. As soon as a plant be- 

 comes terrestrial there must be provided 

 organs (roots or their equivalent) for 

 drawing up from the earth water to re- 

 place what is lost by transpiration, and 

 in all but the simplest forms special con- 

 ducting tissues to facilitate its transport. 

 In the lower types of land plants, the 

 absorptive organs are usually simple hairs 

 (rhizoids), but these are quite inadequate 

 to supply a plant of large size, and conse- 

 quently it is only those terrestrial plants 

 which are provided with a true root system 

 that have succeeded in reaching a large 

 size. Even in the lower terrestrial forms 

 the rhizoids do not monopolize the absorp- 

 tion of water, but many of them are able 

 to absorb water directly through the leaves 

 or through the superficial cells of the thal- 

 lus. While this is especially marked in 

 many mosses and liverworts, which are, so 

 to speak, more or less aquatic in their be- 

 havior toward water, it is by no means 



