336 Kirkwood: Pollen-tube in Cucurbitaceae 



grew toward the pieces from the vicinity of the stigma and were 

 most strongly attracted by ovules ready for fertilization, growing 

 into the micropyle in each case. He concludes that this stimulant 

 is in the nature of a fluid, and in another article has shown that 

 pollen-tubes grow into the stomata of Tradescantia leaves, the in- 

 tercellular spaces of which have been filled with sugar solution- 

 That the pollen-tube is also directed mechanically in its course is 

 maintained by Miyoshi, and Capus states that the mechanical role of 

 the conducting-tissue consists in placing the pollen-tube in relation 

 with the micropyle, and that its economy and simplicity depend on 

 the structure of the ovary. 



That the stimulants which direct the pollen-tube are nutritive in 

 character is quite evident. Dalmer 8 states that pollen-tubes are 

 nourished by amyloid substances secreted by stigma, stylar canal, 

 and the interior of the ovary, though he maintains that the entrance 

 of the pollen-tube into the ovule is a purely mechanical phenome- 

 non. Miyoshi points out that sugar as a stimulant can operate 

 only extra-cellularly, and it is plain that where the course of the 

 pollen-tube is intercellular or through the locular space of the ovary 

 that its growth must be at the expense either of the reserve ma- 

 terials within the tube or of substances elaborated by the tissues in 

 its path. The cytological features of the conducting-tissues of the 

 Cucurbitaceae here under consideration are similar to those of cer- 

 tain nectaries in the abundance of starch and quantity of cytoplasm, 

 appearance of molecular structures, etc. The evidence here points 

 to the secretion of carbohydrates (probably sugar) as the nutritive 

 material. That the pollen-tubes are partial to the more glandular 

 cells is indicated in Cyclanthera. In this case the ovary sometimes 

 shows a considerable locular space at the time of the entrance of 

 the pollen-tubes. The epidermal cells of the placenta are larger 

 and more glandular in appearance than those of the ovary-wall 

 opposite. Though their distance apart is not great, the pollen- 

 tubes always cling to the placenta instead of to the opposite wall, 

 though that likewise is supplied with starch. 



One noticeable feature of the relation between pollen-tube and 

 conducting-tissue in Melothria, Micrampelis and Cyclantliera is the 

 fact that while starch is abundant in the conducting-tissue, none ap- 

 pears in the tube. In Cucurbita Pcpo on the other hand the condi- 



