334 Kirkwood: Pollen-tube in Cucurbitaceae 



course of the tube lies through the tissues, it appears to travel 

 through the intercellular spaces. The destruction of cells in its 

 path seems to be due mostly to mechanical action and not to 

 digestive enzymes, except in the nucellus of Cyclanthera. Fresh 

 ovaries when teased apart showed the pollen-tubes in the stylar 

 canal and passing over various parts of the placental surface. 

 Figures i and 3-5 represent the course of the tube. 



When the surfaces of the placentae are pressed together or 

 against the ovary-wall, the space through which the tubes move is 

 considerably restricted. Under such circumstances they crowd 



* 



aside the cells of the conducting tissue (fig. 11). Sometimes a 

 tube may be seen to have passed under the conducting layer (fig. 

 15). Where the tubes traverse the ovarian space in Melothria 

 they sometimes develop transverse walls or plugs. The manner 

 of their development appears to be the formation of a ring trans- 

 versely which gradually narrows the aperture until it is completely 

 closed. An excessive development in thickness of this wall some- 

 times follows, forming plugs as shown in figure 2 a, b, c. Similar 

 plugs have already been observed by Osterwalder 26 in Aconitum 

 Nape Hits, and by others. 



The pollen-tube proceeds by an approximately direct course to 

 the micropyle, and passes down through the nucellus to the 



embryo-sac. In the apex of the nucellus it usually expands to 



several times its normal diameter in Cyclanthera (figs. 6, 13), and 



from this point sends down a straight and narrow tube to the 



embryo-sac. Sometimes the entire neck of the nucellus is destroyed 



by the dilation of the tube. This expansion of the pollen-tube in 



the apex of the nucellus occurs frequently in Melothria, though it 



was never observed to reach the same proportions as in Cyclanthera. 



In Micrampelis also such conditions were sometimes observed ; here, 



however, they are not the rule but the exception. In such cases, 



both in Melothria and Cyclanthera, the contiguous cells were 

 destroyed. 



That such dilations of the pollen-tube are directly correlated 

 with the occurrence of starch in the surrounding tissues is main- 

 tained by Longo, as above pointed out ; but the behavior of the 

 pollen-tube of Elodea, as described by Wylie, 33 indicates that such 

 conditions are not always the cause of the enlanrments of the tube, 



