300 ESSENTIALS OF BOTANY 
seldom leading straight upward, from very simple forms 
to complex 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 
amply actual relationship, the plants which resemble each 
other most are most closely akin by descent, and (if it were 
not for the fact that countless 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- 
Sul 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 
Algee 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 algze 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 alge 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 
