410 PLANT MORPHOLOGY 



partly through the greater complexity of their vegetative organs, but 

 chiefly by the development of the seed. The development of the seed is a 

 resvilt of three conditions — heterospory, retention of the megaspore within 

 the megasporangium, and formation by the zygote of a dormant embryo. 



Both the bryophytes and pteridophytes are handicapped in their 

 adjustment to the land habit by two basic requirements: (1) In order to 

 reach the egg and effect fertilization, the sperm must swim through water, 

 a handicap persisting from aquatic ancestors. (2) After fertilization has 

 taken place, the embryo must continue its development whether external 

 conditions are favorable or not. 



The seed overcomes both these handicaps. The necessity for external 

 water as a means by which the sperm may reach the egg is obviated by 

 the transportation, through air, of the male gametophyte, inside a pollen 

 grain, to the vicinity of the female gametophyte and the development of a 

 pollen tube through which the male gamete can pass. In this way fer- 

 tilization is made more certain. Following fertilization, the passing of 

 the embryo into a state of dormancy, as well as the formation of a protec- 

 tive seed coat, enables the embryo to live until conditions become favor- 

 able for its continued gro\vth. 



The Flower. The flower is difficult to define because every possible 

 transition exists between a typical strobilus and a typical flower. It is 

 apparent, therefore, that the strobilus is the forerunner of the flower and 

 that the changes involved in passing from the one to the other represent 

 an important evolutionary advance. The perianth probably arose as a 

 protective envelope for the sporophylls, but later came to have additional 

 functions, such as the attraction of insects. In gymnosperms the transfer 

 of pollen is precarious, depending upon the vagaries of the wind. To 

 increase the chances of success, an enormous excess of pollen must be pro- 

 duced, thus involving a tremendous waste. The transfer of pollen by 

 insects, to which the flowers of most angiosperms are highly adapted, is 

 more certain and consequently much less w^asteful than wind-pollination. 

 Probably the most important factor responsible for the evolutionary prog- 

 ress made by the angiosperms has been the development of the flower in 

 adaptation to insect pollination. 



Associated with the development of the flower and its specialization for 

 insect pollination has been the enclosure of the ovules. Originally the 

 carpels of seed plants must have been leaf-like structures bearing marginal 

 ovules, a condition preserved in such existing gymnosperms as Cycas 

 revoluta. The change from the open carpel with its ovules freely exposed 

 to the closed carpel with its ovules inside a cavity marks another great 

 advance which the angiosperms have made over the gymnosperms. This 

 change led to the development of the fruit. 



Interrelationships. The fossil record gives no evidence as to which of 

 the two great Paleozoic groups of gymnosperms — the Cycadofilicales and 



