XV NATURE OF THE ALTERNATION OF GENERATIONS 565 



Liverwort and any known Alga forbids the assumption of any 

 but a very remote connection Ijetween them. 



In all typical Liverworts which are characteristically terres- 

 trial plants, in addition to the rhizoids for absorbing w^ater, 

 there is also a more or less perfect cutinisation of the superficial 

 cells which materially checks the loss of water from transpira- 

 tion. In addition to this there arc often special provisions for 

 protecting the plants from injury by drought. Most species 

 have mucilage secreting organs of some kind, and the hairs and 

 scales frequently developed upon the plant are usually associated 

 with water storage. Like some Alg?e, certain Liverworts can 

 become dried up without injury, reviving promptly when sup- 

 plied wdth water. Less frequently special tubers are formed, 

 these being especially marked in some species from dry regions, 

 like those about the Mediterranean or in Southern California. 



In passing from an aquatic to a terrestrial habitat, another 

 change of structure must be noted, namely, the development of 

 mechanical tissues for giving the plant body the necessary sup- 

 port in the much rarer medium of the atmosphere. In studying 

 the evolution of the gametophyte in the Bryophytes, it becomes 

 at once evident that the development of mechanical tissues is 

 largely obviated in the lower types by their never attempting to 

 stand upright, but they lie prostrate upon the ground as we may 

 assume w^as done by their algal prototypes. This prostrate 

 position, W'hile doing away with the necessity for skeletal tissues 

 also has the advantage of offering a much larger surface for 

 the development of the rhizoids, and also exposes a smaller sur- 

 face directly to the air and consequently reduces the loss of water 

 by evaporation. Most of the lower HepaticDe and all the 

 Anthocerotes have retained this primitive type of gametophyte. 

 In the Mosses, however, the prostrate thallus is replaced by a 

 definite leafy axis, which is often upright and may develop a 

 fairly complete system of skeletal tissues. This type realises 

 its most perfect expression in such large Mosses as Polytrichnm 

 and Dawsonia. We find in these that in addition to the 

 mechanical elements, there are also* water-conducting tissues, 

 comparable to the tracheary tissue of the vascular plants, 

 although in one case we have to do with gametophytic struc- 

 tures, in the other with sporophytic ones. In these large 

 Mosses, the rhizoids are multicellular, and may be twisted into 



