392 THE SPERMATOPHYTES 



tubes would be expected to become more and more specialized 

 as conditions arose which led to the final retention of the rnega- 

 spore within the megasporangium, and at last they assumed pro- 

 nounced parasitic habits. 



With the parasitic habits of the pollen tube established it is 

 not difficult to imagine the gradual adjustment of the peculiar- 

 ities of pollination to those of ovule formation. It seems prob- 

 able that the earliest forms of pollen tubes carried motile sperms 

 to the embryo sac l (for motile sperms are even now present in 

 the cycads and Ginkgo, Sec. 348), but later the complex struc- 

 ture of the sperm degenerated, with that of the whole male 

 gametophyte, until the sperm nuclei became practically all that 

 was left to represent the male gametes of the pteridophytes, 

 bryophytes, and alga3. The simplification of the sperm and 

 egg in the spermatophytes does not, however, affect the signifi- 

 cance of these sexual elements, because it is known that the 

 nuclei are the most essential structures of gametes. 



Thus the peculiarities of the ovule and the pollen tube prob- 

 ably developed side by side, adjusting themselves to one another 

 until the complex phenomena of pollination became established. 

 These processes are relatively simple in the gymnosperms, where 

 the pollen is applied directly to the ovule ; but in the angiosperms 

 a new feature was introduced when carpels, or groups of carpels, 

 frequently with adjacent tissue of the stem, developed the ovule 

 cases (ovaries). Yet it is not very difficult to understand how 

 they may have arisen, for the same principles of protecting 

 the megaspore (embryo sac) and providing for the germination 



1 In certain fossil groups (Pteridospermce) , intermediate between the 

 pteridophytes and spermatophytes, the evidence indicates that motile sperms 

 were discharged into large pollen chambers filled with water, into which 

 the necks of the archegonia opened so that the sperms were able to swim 

 directly to the eggs. The pollen tube was probably at first an absorbing 

 organ, or haustorium (as in the cycads and Gink r /o to-day, Sec. 348), pene- 

 trating the tissue of the ovule to obtain nourishment for the parasitic male 

 gametophyte. Later, with the disappearance of the pollen chamber and 

 motile sperms, the pollen tube took on the added function of carrying the 

 sperm nuclei directly to the embryo sac. 



