PHYLOGENY 



which develop into male gametophytes. In spite of their names, the two 

 types may be of equal size, or the microspores may actually be the larger. 

 The cones in which the spores are formed consist of specialized sporo- 

 phylls spirally arranged about a central axis. They are thus modified leaf 

 clusters. The microspores divide while still within the spore wall to form 

 the male gametophyte. The pollen is shed in great numbers while in the 

 four-cell stage, and is carried by the wind, sometimes for great distances. 

 Some of the pollen will reach female cones and become stuck in a sticky 

 fluid exuded by the ovules, complex structures which include the female 

 gametophyte. The pollen produces a tube-like growth which enters the 

 ovule. Down this pollen tube pass the two sperm nuclei, one of which 

 fertilizes the egg. The developing embryo (sporophyte) is pushed into 

 the mass of the female gametophyte, which is now called the endosperm 

 and which serves as nutritive material for the embryo. This endosperm 

 is, in turn, surrounded by a seed coat, which is actually a part of the 

 parent sporophyte. The seed is shed, and if it falls on favorable ground 

 the seedling may develop into a mature sporophyte. 



Some significant aspects of this reproductive cycle should be indicated. 

 For the first time in the phylogenetic series, reproduction is independent 

 of water. The pollen is carried to the female cones by wind, and the sperm 

 —mere nuclei rather than flagellate cells— are carried to the egg by the 

 protoplasmic pollen tube. Both the male and female gametophytes are 

 reduced to minute structures consisting of only a few cells, and thus the 

 relative predominance of the sporophyte in the life cycle has become 

 great. The gametophytes lack chlorophyll and are completely dependent 

 upon the sporophyte. Finally, the seed, a new structure in the phylo- 

 genetic series, consists of an embryo (sporophyte) contained within the 

 endosperm ( gametophvte ) which is in turn contained within a seed coat 

 which is sporophvtic tissue of the parental generation. Thus the appear- 

 ance is very much as though the embryo sporophyte were produced di- 

 rectly by the parent sporophyte, with the gametophytes being simply 

 organs of the parent sporophyte. Only comparison to lower plants reveals 

 the true situation. 



The Flowering Plants. The third and final class of the subphylum 

 Pteropsida is the class Angiospermae, the flowering plants. This is much 

 the dominant class of th(> world's flora today, including on the order of 

 1(),0()() genera and 195,000 species. The variety of the angiosperms is un- 

 limited, ranging from great trees to grass; generally they are land plants, 

 but they have become adapted to almost every available habitat, includ- 

 ing marine; and while they are typically free-living green plants, not a 

 few have becornt^ parasitic and some are saproplntic-. 



The angiosperms share the major characters of the gN ninospenus, being 

 diilercMitiated into true roots, stems, and leaves, and reproducing by means 

 of true seeds. They have a highly developed vascular system. Fertilization 

 is by means of j;)()llen which are indepcuident ol water. The sporophyte 

 is much tlie dominant generation, the ganiet()[)ln te being minute and 

 complc^telv dependent, in addition, th(> s]")orangia of the angios]")(Mms are 

 within flowers, which are modified cones surrounded by modified and 



122 



