January 16, 1903.] 



SCIENCE. 



99 



over its aquatic competitors, and thus the 

 terrestrial habit was established. 



Some liverworts and mosses may reach 

 considerable size, a foot or more in length 

 in a few cases. They also exhibit a cer- 

 tain amount of specialization, correspond- 

 ing to the requirements of the terrestrial 

 environment. Well-developed leaves are 

 present in nearly all true mosses, and in 

 many liverworts, and in one order of the 

 latter, the Marchantiales, the plant body, 

 while retaining its thallose character, de- 

 velops a complicated assimilative tissue, 

 with stomata of a peculiar type not found 

 elsewhere. In the upright forms' mechan- 

 ical tissues are developed, and in the true 

 mosses there is present in the leafy shoots 

 a central strand of conducting tissue, com- 

 parable to the vascular bundles found in 

 the sporophytes of the vascular plants. In- 

 deed the analogies existing between the 

 leafy moss-shoot and the sporophytic 

 shoots of the vascular plants are suffi- 

 ciently obvious. 



No existing bryophytes have succeeded 

 in reaching any but the most modest di- 

 mensions. All the larger forms either are 

 prostrate or grow in dense tufts, offering 

 mutual support to the leafy shoots. In- 

 deed no moss seems to have quite solved 

 the problem of a self-supporting upright 

 leaf -supporting axis. Neither have they 

 successfully solved the problem of an ade- 

 quate water supply, to compensate for loss 

 of water by transpiration, and this of 

 course is closely associated with the limit 

 of size which the plant-body could assume. 

 Given an unlimited water supply, and a 

 plant, even of low organization, may attain 

 very large dimensions, as we see in the 

 giant kelps. Those plants, although in 

 many respects of very low rank, neverthe- 

 less may reach hundreds of feet in length, 

 and develop specialized tissues, curiously 

 suggesting those of the highly organized 



land plants. These giant seaweeds ab- 

 sorb water throughout their whole super- 

 ficial area, and there is no loss of water 

 by transpiration ; but for a terrestrial plant 

 to reach a large size there must be ade- 

 quate means for absorbing water from the 

 soil, and for transporting it expeditiously 

 through the plant to those places where 

 water is being lost through transpiration. 

 In the highest terrestrial plants, the 

 'vascular' plants, we meet fii-st with a per- 

 fect system of water-conducting tissue. 

 This is the woody portion of the fibre- vascu- 

 lar bundles, composed of the characteristic 

 tracheary tissue, first encountered in the 

 ferns, and common to all the higher plants. 



OEIGIN OP THE SPOROPHYTE. 



Among the lower terrestrial plants, the 

 Archegoniata, which comprise the mosses 

 and ferns, a very marked characteristic is 

 the 'alternation of generations.' By this 

 is meant that in its development the plant 

 passes through two very different phases, 

 a sexual and a non-sexual one. This is per- 

 haps best seen in the ferns. The spore of 

 the fern, on germination, gives rise not to 

 the leafy fern plant, but to a much simpler 

 plant much like b small liverwort, upon 

 which the sexual reproductive organs, the 

 archegonium and antheridium, are borne. 

 This sexual plant is known as the gameto- 

 phyte. Within the archegonium is borne 

 the egg-cell or ovum, which, after being 

 fertilized, ultimately produces the leafy 

 fern plant, or ' sporophyte, ' from its pro- 

 ducing the spores, or non-sexual reproduc- 

 tive bodies. 



Among the lower Archegoniates, the 

 gametoph3i:e is relatively much more im- 

 portant, and the sporophyte is never an 

 independent plant, as it is in the ferns, but 

 always remains to a greater or less extent 

 dependent upon the gametophyte for its 

 existence. 



