MICHIGAN ACADEMY OF SCIENCE. 



335 



Figure 17. Pinus sylvestris. 



5, 7, 8, 1. Various stages of pollen tube elongation are here illustrated 

 as well as one case with two tubes (Fig. 17-6). All seem to. show the 

 generative and tube cell stage. No evidence of a division of the genera- 

 tive cell into the stalk cell and body cell was found, though the tube 

 compares almost exactly in length to that shown in Fig. 16-L from 

 Coulter and Chamberlain and the tube nucleus is in the expected posi- 

 tion for this stage. It is to be noted that Coulter and Chamberlain's 

 figure (Fig. 16-L) records the condition after four or five months' devel- 

 opment within the nucellus of the ovule. The position of the tube 

 nucellus, however, is responsible for the tube elongation and is not to be 

 considered as evidence of the nuclear condition in the spore. 



It has been assumed that tube development was a chemotropic 

 response ; the stimulus arising from a mucilagenous substance secreted 

 by the nucellus — the so-called "pollination-drop" — or from contact with 

 the nucellus. Just what our observation may mean is problematical. 

 That the primitive function of the pollen tube in the Gymnosperms was 

 haustorial and that its present function in Siphonogamy derived is, I 

 believe, generally acknowledged. Whether this anomalous development 

 of the pollen tube within the microsporangium is to be interpreted as 

 recapitulative or as a response to some unusual intra-microsporangial 

 nutritive condition can only be answered by further investigation. In 

 any case it is evident tliat the chemotropic stimulus of the nucellus or 

 pollination-drop is not essential for the development of the pollen tube 

 in Pinus sylvestris. 



LITERATURE CITED. 



^Coulter, J. M., and Chamberlain, C. J. Morphology of Gymnosperms. 

 1910. 



