THE SEED PLANTS 305 
changed greatly from the oldest simple plants. While, there- 
fore, we compare one living group with another, we must keep 
in mind the fact that a higher group of living plants has not 
necessarily developed from one of the lower living groups, 
but rather that in past ages a common ancestry gave rise 
to both. The lower group has probably changed less than 
the higher one. It is like two streams of water that have 
their source on the top of the same mountain; though their 
source is essentially the same, the conditions under which 
they flow may make the two rivers quite unlike when they 
reach the valleys below. 
290. Brief summary of the groups. The thallophytes con- 
sist of simple plants, some of which (the alge) possess chloro- 
phyll, by means of which they manufacture their own foods. 
Other thallophytes (the fungi), being without chlorophyll, 
cannot make their own foods and are dependent. Depend- 
ency is expressed in types of parasitism and saprophytism, 
which are often of importance to other living things. These 
simple plants are prostrate and are not differentiated into 
roots, stems, and leaves. While reproduction is simple in 
this first group, in some plants we find specialized sex organs, 
with sperms and eggs for the formation of sex spores. 
In the bryophytes the moss exhibits a much more complex 
type of plant. It has an alga-like stage; then it sends up leafy 
shoots, which expose chlorophyll to the light in a better way 
than appeared in the thallophytes; these shoots also bear 
highly differentiated sex organs. From the sex spore there 
grows a special asexual spore-forming stalk and capsule, from 
1 No attempt is made at this time to give a complete summary of all the 
important characters of the groups studied; the aim has been rather to 
state only the most important things which will cause each group to stand 
out with some individuality and at the same time give it some relation to the 
series of groups as a whole. Neither is any attempt made in this book to 
present the evolutionary series of plants. The groups are presented so that 
the increasing complexity is apparent, but the close evolutionary connections 
are omitted, and the emphasis that is sometimes placed upon evolution is 
here placed upon securing an elementary idea of the kinds of common plants 
that are found in each of the great groups. 
