NATURE OF PLANTS 121 



solutions is to nourish the microspores. It is cvick-iii that tliese 

 minute dust-Hke cells can not contain sufficient f(xxl to bring 

 about any considerable growth. It is noteworthy tliat 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 or in the presence of such substances 

 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 them- 

 selves. 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 cell respectively (Fig. 79, A, B). 

 The latter cell divides once, forming two cells, called the male 

 gametes (Fig. 79, C). This stage in the germination 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 wa\- throut;h 

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

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

 time the two male gametes have been carried down by the 

 streaming movement of the cytoplasm 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 !)>■ 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 

 growth down into the female gametophyte can only be accounted 

 for by some chemical stimulus that is located in the ovule and 



