284 BOTANY: PRINCIPLES AND PROBLEMS 



the majority of the red algae. Three types of plants, similar in 

 vegetative structure but differing in their reproductive organs, 

 may be distinguished: Male plants, producing antheridia only; 

 female plants, producing procarps only; and sexless tetrasporic 

 plants, producing sporangia in each of which are four asexual 

 tetraspores. The procarp is somewhat more complex than in 

 Nemalion since it contains a group of auxiliary cells. Fertili- 

 zation, however, occurs in much the same way, a sperm coming in 

 contact with the trichogyne, entering the carpogonium, and 

 fusing with the female nucleus. Some rather complicated 

 fusions now take place between the fertilized carpogonium and the 

 auxiHary cells, as a result of which sixty or more carpospores are 

 produced. An envelope of sterile cells grows up from the base 

 and encloses this spore mass, forming the cystocarp. 



Experiment has established the fact that these carpospores 

 produce only tetrasporic plants, and that the tetraspores in turn 

 produce only male or female plants. Thus a regular alternation 

 of sexual and non-sexual individuals is set up. Cytological 

 examination has further proven that the tetrasporic plants have 

 twice the number of chromosomes possessed by the sexual plants, 

 and there seems no reason to doubt that we are dealing here 

 with a true alternation of generations, the tetrasporic plant (plus 

 the cystocarp) being a sporophyte, and the sexual plants, gameto- 

 phytes. This is the more remarkable since all the plants are 

 perfectly similar in their vegetative structures. 



The red algae are very evidently a specialized class and 

 although they have reached a marked degree of complexity, they 

 apparently have not been the ancestors of anything higher up in 

 the evolutionary series. 



THE FUNGI 



The other great group of the thallophytes are the fungi, 

 which are distinguished from algae by the absence of chlorophyll. 

 Their (Consequent inability to manufacture food therefore compels 

 fungi to live either as saprophytes or as parasites. Like the algae, 

 this is a heterogeneous group and many of its members seem more 

 closely related to certain groups of algae than to other fungi, but 

 as a matter of convenience and custom, and in the absence of any 

 widely accepted "natural" classification of the thallophytes, they 

 will all be considered together. This immense arraj^ of lowly 

 plants is much more numerous in specie^ than the algae and 



