304 



STRUCTURAL BOTANY 



down this distinction, for they proved that in some 

 Gynmosperms true motile spermatozoids are formed. 



The plants in 

 which this im- 

 portant discovery 

 was first made are 

 the Maidenhair 

 Tree (Ginkyo 

 biloba), a tree of 

 an ancient type, 

 nearly related 

 to the Coniferae, 

 and Gycas revo- 

 luta (see p. 35). 

 In both these 



FIG. 118.Ginkgo Uloba. A, pair of generative 

 cells in the pollen-tube. On the outer side of 

 each cell a spiral coil is seen in connection with 

 the nucleus. B, generative cell, showing the 

 spiral spermatozoid in surface view, x 225. 

 From a preparation by Dr. Hirase. (R. S. ) 



cases (to which others have now been added) a pollen-tube 

 is formed, which enters the uucellus, but does not reach the 



archegonia, and serves 

 chiefly to anchor the 

 pollen - grain in the 

 right position. Two 

 ^generative cells are 

 formed in the usual way, 

 but each of these gives 

 rise to a large, spirally - 

 coiled spermatozoid, 

 with numerous cilia 

 (see Figs. 118 and 11 9, 

 which have been 

 sketched from the origi- 

 nal preparations, kindly 

 sent by Prof. Ikeno and Dr. Hirase). The spermatozoids 

 break out from the pollen-tube, and by their own active 



FIG. 119. Cycas revoluta. Pair of genera- 

 tive cells from a pollen-tube, showing 

 the spirally - coiled spermatozoids, sur- 

 rounded by the protoplasm of the cell. 

 The fine striation overlying the spiral 

 coil indicates the cilia, x 190. From 

 a preparation by Prof. Ikeno. (R. S. ) 



