NATURE OF PLANTS 107 



the strength and nature of the sugar solutions varies in different 

 stigmas and that the microspores are often able to grow only 

 in solutions of such strength as are found on the stigmas of their 

 own kind of plant. So it results that the microspores are often 

 unable to grow when carried to the stigmas of a different kind 

 of plant from themselves. It will also be noted later in the work 

 that the flower may be so developed that the microspore cannot 

 be transferred to the stigma of its own flower but only to that 

 of another. This is brought about in various ways, as by the 

 shedding of the microspores before the stigmas are mature, etc. 

 This transfer of the microspores of one flower to the stigmas 

 of another flower is called crossing, the significance of which 

 will appear directly. The germination of the microspore results 

 in the division of its nucleus and the formation of a large and 

 a small cell known as the tube and antheridial cells respectively 

 (Fig. 79, A, B). The latter cell divides once, forming two cells, 

 called the male gametes (Fig. 79, C). This stage in the germina- 

 tion of the microspore is reached either in its sporangium or 

 after being transferred to the stigma. Nourished by the foods 

 of the stigma the microspore continues its growth, the tube cell 

 ruptures the outer wall of the spore and forms a tube-like growth, 

 This tube cell grows down between the cells of the style into the 

 cavity of the ovary where it usually curves out and enters the 

 ovule by way of the micropyle. It now works its way through 

 the tissues of the nucellus to the female gametophyte, into the 

 cavity of which it enters after dissolving the wall. In the mean- 

 time the two male gametes have moved down to the end of the 

 tube cell as it enters the female gametophyte (Fig. 79, D). The 

 question will naturally be asked, what directs the peculiar growth 

 of this minute plant? In the first place the tube cell is repelled 

 by the oxygen of the atmosphere, so as soon as it appears, it is 

 directed away from the atmosphere into the stigma. Its course 

 down the style to the ovary is very largely controlled by lines of 

 loose tissue through which it can easily work its way and also 

 by foods which are deposited in the cells just ahead of the tube 

 cell so that it is led along rather straight pathways to the ovary. 

 The curving out of the tube to the micropyle and its subsequent 



