THE GYMNOSPERMS 



401 



On the germination of this and the establishment of the Zamia 

 plant the life history is complete. 



550. Gingko. Gingko (Gingko biloba) is a very interest- 

 ing tree. Tt is a native of China and is now widely grown in 



Fig. 386. 



Sperms of zamia in 

 pollen tube; Pg, pollen 

 grain; aa, sperms. 

 (After Webber.) 



Fig. 387- 



Sperm of zamia, show- 

 ing spiral row of cilia. 

 (After Webber.) 



Fig. 385- 



Zamia. Germinating pollen 

 grain in more advanced stage, 

 the pollen tube with nucleus of 

 tube cell not shown. The cen- 

 tral cell has divided into two 

 cells; the one at the left is the 

 generative or body cell preparing 

 to divide into the two sperms. 

 The prothallia cell at the left is 

 growing out into the other cell 

 (stalk cell) derived from the 

 division of the central cell. 



Europe, America, and other countries, as an ornamental tree. 

 It is the sole survivor of a group of plants which were very 

 abundant in geological times. The leaves are triangular, radi- 

 ately veined, and resemble in form the pinnules of the maiden 

 hair fern (Adiantum). The large 

 spore cases form a fleshy fruit, about 

 the size of a plum, with a soft exocarp 

 and a stony endocarp, the meat within 

 being the female prothallium or endo- 

 sperm. There are motile sperms, 

 and the life history is similar to that 

 of the cycads. 

 551. Comparative review of the Fig - 388 - 



_ . _,. Gingko biloba, end of stem with 



GymriOSpermS.* The lowest Gym- spray of leaves and two fruits. 



nosperms are represented by the cycads. They bear certain 

 resemblances to the ferns, which indicate that they have had 

 their origin from some fern-like ancestors which are not now 

 known. These resemblances are as follows: 

 First, the form of the leaves. 



* For reference. 

 26 



