300 ESSENTIALS OF BOTANY 



seldom leading straight upward, from very simple forms 

 to com})lex ones. The humblest thallophytes are merely 

 single cells of microscopic size. Class after class shows 

 an increase in complexity of structure and of function 

 until the most perfectly organized plants are met with 

 among the dicotyledonous angiosperms. During the lat- 

 ter half of the nineteenth century it first became evident 

 to botanists that among plants deep-seated resemblances 

 imjjily actual relationship ^ the plants which resejnble each 

 other most are most closely akin by descend, and (if it ivere 

 not for the fact that comdless forms of plant life have wholly 

 disappeared) the whole plant kingdom might have the rela- 

 tionships of its members worked out by a sufficiently care- 

 ful study of the life histories of individual plants and the 

 likenesses and differences of the several groups which make 

 up the system of classification. 



381. Development of the Plant from the Spore in Green 

 Algse and Mosses. — The course which the forms of plant 

 life have followed in their successive appearances on the 

 earth may be traced by the application of the principle 

 stated in Sect. 379. 



Such algse as the pond-scums produce spores which give 

 rise directly to plants like the parent. 



A moss-spore in germination produces a thread-like pro- 

 tonema which appears very similar to algse of the pond- 

 scum sort. This at length develops into a plant with 

 stem and leaves, — the sexual generation of the moss. 

 The fertilized archegonium matures into a sporophyte 

 which is the alternate non-sexual generation. This is 

 attached to the moss-plant or gametophyte, but is an im- 

 portant new organism. In the moss the sexual generation 

 is the larger and more complex of the two, the non-sexual 



