Evolution of Plants 321 



carpogonia that project amid cellular threads. The fertilized 

 egg gives rise to dense clusters of carpospore cells. 



In transition to the marine groups the formation of tetra- 

 spores amongst these is an almost invariable feature. The 

 clusters of spermatia are produced as prominent terminal 

 masses on special branches. The increasingly complex carpo- 

 gone with subsequently formed cystocarp and enclosed carpo- 

 spores have been so often described in text-books that the 

 accompanying figures will suffice to illustrate them (Fig. 10). 



A short summary may now be made of the main details of 

 the present chapter. In connection with arguments advanced 

 already here the position has been taken that living organ- 

 isms originated during the archsean age, amid warm thermal 

 waters, and by gradual colloid synthesis. The primitive 

 organisms were probably colorless bacterial forms similar to 

 the sulphur and thermal bacteria, and from these by slow 

 evolution the unicellular Blue-green Algse originated. The 

 latter consisted of a cylindrical zone of pigmented protoplasmic 

 •substance, which surrounded a central mass of clear proto- 

 plasm or synthesizing substance. The pigment zone seems 

 to have functioned equally as an absorber, a condenser, a 

 transformer, and a storing area for energizing food material. 



The cyanophyceous organisms probably originated from 

 more primitive bacterial ancestors first in thermal fresh-water 

 areas, and at temperatures of 75-50° C. We have accepted 

 it that 41 existing species which still live under such conditions 

 are probable direct descendants of these. By acclimatization 

 to cooling terrestrial conditions in fresh-water, on moist sur- 

 faces, or in shady moist places, and later in the sea, a wealth 

 of species probably evolved during mid-archsean times that 

 became the ancestors of all green plants, while the prepon- 

 derating wealth of species now living in fresh-water or moist 

 land areas, as compared with the much smaller number of 

 marine species, seems to be a correct index to their primitive 

 environment. 



From green or reddish green unicellular Cyanophycese we 

 have traced what are probably the simplest nucleate green 

 algae hke Pleurococcus and Tetrasporay while from olive-green 

 11 



