140 BOTANY. 



We have already examined in some detail the structure of the macro- 

 sporangium or ovule. In the full-grown ovule the macrospore, which in 

 the seed plants is generally known as the "embryo sac," is completely 

 filled with the prothallium or "endosperm." In the upper part of the 

 prothallium several large archegonia are formed in much the same way 

 as in the pteridophytes. The egg cell is very large, and appears of a yel- 

 lowish color, and filled with large drops that give it a peculiar aspect. 

 There is a large nucleus, but it is not always readily distinguished from 

 the other contents of the egg cell. The neck of the archegonium is quite 

 long, but does not project above the surface of the prothallium (Fig. 77, H). 



The pollen spores are produced in great numbers, and many 

 of them fall upon the female flowers, which when ready for 

 pollination have the scales somewhat separated. The pollen 

 spores now sift down to the base of the scales, and finally 

 reach the opening of the ovule, where they germinate. No 

 spermatozoids are produced, the seed plants differing in this 

 respect from all pteridophytes. The pollen spore bursts its 

 outer coat, and sends out a tube which penetrates for some 

 distance into the tissue of the ovule, acting very much as a 

 parasitic fungus would do, and growing at the expense of the 

 tissue through which it grows. After a time growth ceases, 

 and is not resumed until the development of the female pro- 

 thallium and archegonia is nearly complete, which does not 

 occur until more than a year from the time the pollen spore 

 first reaches the ovule. Finally the pollen tube penetrates 

 down to and through the open neck of the archegonium, until 

 it comes in contact with the egg cell. These stages can only 

 be seen by careful sections through a number of ripe ovules, 

 but the track of the pollen tube is usually easy to follow, as 

 the cells along it are often brown and apparently dead (Fig. 

 77, G). 



CLASSIFICATION OF THE GYMNOS PERMS. 



There are three classes of the gymnosperms : I., cycads 

 (Cycadece)-, II., conifers (Coniferce), III., joint firs (Gnetacece). 

 All of the gymnosperms of the northern United States belong 



