SEXUALITY OF PLANTS 



47 



Seed-Plants really were evolved from organisms which 

 were fertilized as Ferns are, is shown by facts recently 

 discovered. For certain primitive Seed-Plants have been 

 found to have motile spermatozoids (Fig. 29). But they 

 move only in the limited sphere of a small volume of fluid 

 within the ovule. These Plants have been driven by the 

 exigencies of their land-habit to secrete the medium in 

 which their spermatozoids move. & - 



For Land-Plants such an archaic 

 method so artificially maintained is 

 clearly unpractical. The Cycads 

 and the Maiden-Hair Tree that 

 show it are rightly regarded as sur- 

 vivals, whose conservatism has 

 almost cost them their lives. The 

 rush of Evolution of Land-Plants, 

 with a more practical method of 

 fertilization, has passed them by. 

 But they at least survive to tell the 



story of Descent, and to point its ( about m After Webber> 

 moral. 



The transition from water to land has thus profoundly 

 affected the mechanism of sexuality in Plants, without 

 altering the essential features of the process. Syngamy 

 is still a fusion of two sexual cells of more or less distinct 

 origin to form one, which is the starting point for a new 

 individual. Witnessing the consequences of the change 

 of medium, the mind is impressed by a sense of the 

 vulnerability of primordial cells exposed to the air. In 

 water the risk is not great. The gametes may be voided 

 directly into water, as they are in Fuciis. Syngamy 

 is then carried out quite apart from the parent, and the 

 new individual is independent of parental nursing. But 



FIG. 29. 



End of Pollen-tube of Zainia, 

 showing the vegetative cell (v), 

 sterile cell (s), and two sperma- 

 tozoids. (a) before movement 

 has commenced ; (b) after the 

 beginning of ciliary motion. 



t 

 from Strasburger. 



