THE SEED PLANTS 305 



changed greatly from the oldest simple plants. While, there- 

 fore, \ve 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. 1 The thallophytes con- 

 sist of simple plants, some of which (the alg<e) 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. 



