A MANUAL OF BOTANY 147 



value ot the same, and the leatures that lead up to and re- 

 semble the cone-bearing trees (the next study). All these 

 features are a [)art of the evolution of plants from lower to 

 higher forms, and more or less clearly point out (what is 

 the belief commonly held among- scientific men) that all 

 the complex structures of the highest fiower plants have 

 been slowly evolved in the long past history of our earth, 

 from lower and simpler forms, and these in like manner 

 from yet simpler, until at length we reach the primordial 

 cell as in pleurococcus, or some similar structure. All the 

 factors entering into these changes of form are not clearly 

 known, but the effect of environment, the survival of useful 

 and beneficial qualities, the influence of heredity, and, above 

 all, the tendency of the Uvinri principle to variations, small 

 and great, have much to do with the changes wrought. 

 This is a very difficult subject, and it is only hinted at here 

 that you may know a little about the theory that men use 

 to explain the great diversity of plant forms that at the 

 same time are connected by features of structure and repro- 

 duction. 



SFXTION V. THE SEED PLANTS 



Class I. Gymnosperms (Conifers, "Evergreens") 

 A. INTRODUCTORY 



We have now finished our examination of the plants of 

 the three branches (Thallus, Moss, and Fern) below the 

 Seed Plants, the last and highest of these great groups. 

 These lower plants have often been called Jlowerleas plants, 

 not possessing flowers. Another term commonly applied is 

 cryptograms, which signifies a hidden or obscure reproduc- 

 tion. A better understanding of them is, however, given by 

 the use of the three terms above. 



The Seed Plants, while in many features very diverse, 

 unite in the production of seeds, structui'es diiferent from, 



