12 A TEXTBOOK OF BOTANY [Ca. II 



situations. They are undoubtedly descended, as shown by 

 many resemblances in structure, from the Algae ; and so close 

 are their relationships that, /rom the point of view of classi- 

 fication, the two groups are properly included in one, called 

 Thallophytes, though in practice it is convenient to treat 

 them separately. 



5. The Seaweeds and their kin, called scientifically 

 Alg^e, comprise not only the red and brown seaweeds and 

 "sea mosses" (which are green underneath those colors), 

 but likewise many green kinds both of salt and fresh water. 

 They live mostly under water, make their own food in their 

 fronds, have diverse shapes with different habits, and re- 

 produce both by simple spores and sexual stages. They are 

 the simplest and most ancient of the leading groups, and 

 the one from which the others are descended. 



Algse, Fungi, Bryophytes, and Pteridophytes are often 

 called collectively Cryptogams, because their reproduction 

 was once thought obscure, while the Spermatophytes are 

 called Phanerogams, because their reproduction, through 

 flowers, was considered evident. 



It is the primary aim of science to discover, analyze, de- 

 scribe, and classify the elemental facts of nature. It is a 

 secondary aim to explain phenomena with which the facts are 

 connected, though to all except specialists the explanations 

 are hardly inferior in interest to the facts themselves. In 

 this book, while the description of fact always comes first, 

 explanations follow promptly after. The explanations of 

 the phenomena exhibited by living plants fall under four 

 categories. First, a great many features, especially those 

 connected with the obvious fitness of form and structure 

 to functions and habits, are best explained, in the opinion 

 of a majority of biologists, as result of a process of gradual 

 adaptation of the modifiable plant to the unmodifiable 

 physical surroundings during the course of evolution. Second, 

 other features are clearly survivals, of no other present sig- 



