GROWTH OF POLLEN TUBE 353 



all this cell destruction gives ample occasion for the forma- 

 tion of the wound hormones postulated by Haberlandt and 

 regarded by him as responsible for the stimulus leading to 

 development of the ovum which accompanies fertilisation. 



In a number of plants, notably among the CupuUferae, 

 the pollen tube, instead of entering the micropyle, passes 

 up the funicle and enters the ovule by the chalaza — chalazo- 

 gamic as opposed to porogamtc fertiUsation. There are 

 great variations in the exact route followed by the pollen 

 tube in these cases. The cause and significance of this 

 procedure are obscure. One other abnormal case may be 

 noted. In cleistogamic flowers, e.g. in the sweet violet, 

 where self-pollination occurs without the flower opening, 

 the pollen grains may germinate in the pollen sac and the 

 tubes pierce the walls of anther and ovary. 



In the gymnosperms the conditions are quite different. 

 The ovule is naked and the pollen is received directly by the 

 micropyle. The pine may be taken as an example. At the 

 time when the pollen is shed in spring, a year after the 

 microspores are fully formed, a drop of mucilaginous fluid 

 is excreted by the nucellus through the micropyle, and in 

 this the pollen is caught. The fluid dries up, retracts, and 

 the pollen is pulled through the micropyle, where it 

 germinates and begins to penetrate the nucellus ; slow growth 

 continues till checked by winter. Only in the following 

 spring do the tubes reach the egg cells, several of which may 

 be fertilised. In gymnosperms, other than the conifers, 

 while the growth of the pollen tube is always slow, fertilisa- 

 tion takes place within a few months of pollination. In 

 the Cycads, in Ginkgo, and in the Gnetales a pollen chamber 

 is formed at the tip of the nucellus, and in Ginkgo it may 

 even project beyond the micropyle. In the Gnetales the 

 pollen is received by a long protruding micropylar tube. 

 In the Cycads and in Ginkgo the sperms are ciUate, the 

 last case of active sperms in the plant kingdom. In the 

 Cycads a branched pollen tube penetrates the nucellus, but 

 functions only as a haustorium. This suggests that the 

 primary function of the pollen tube was nutritive, it was 



2 A 



