Kirkwood : Pollen-tube in Cucurbitaceae 335 



for in this case they occur in the ovarian cavity. This has already 

 been suggested by Lloyd 16 in discussing the pollen-tube in the 

 Cucurbitaceae and Rubiaceae. 



m 



In the plants here under consideration the nucellus is practi- 

 cally devoid of starch, which is abundant in the integuments of 

 Micrampelis and Cyclanthera, though not so abundant in Melotliria. 



The anatomical character of the conducting-tissue is a feature 

 of some interest. In all cases examined a continuous conducting- 

 tissue reaches from the stigma to the micropyle. In Micrampelis 

 the neck of the nucellus pushes through the micropyle, and comes 

 out even with the top of the integuments. Practically the same 

 conditions exist in Cyclanthera, but in Melotliria the apex of the 

 nucellus is usually overtopped by the integuments. In the three 

 genera just mentioned the conducting-tissue consists of a single 

 layer of cells, the epidermis covering the placentae and lining the 

 stylar canal. This layer appears to be the only one serving directly 

 as conducting-tissue. On the stigma the starch is restricted to the 

 subepidermal tissue, none appearing in the more densely plasmatic 

 cells of the epidermis. The glandular character of the epidermal 

 cells is quite apparent both on the stigma and in the interior (figs. 

 9, i i, 14). In the interior the cells in the path of the pollen-tubes 

 show indications of activity, by the dense aggregation of the cyto- 

 plasm next the exposed side and surrounding large and conspicu- 

 ous nuclei. 



Considerable effort has been made to determine the directive 

 force controlling the movements of the pollen-tube. We may note 

 in passing that Nawaschin, 24 Zinger 31 and some others have inter- 

 preted the endotropic and ectotropic behavior of pollen-tubes as of 

 phylogenetic significance. But evidence at present available seems 

 to point to physiological processes and mechanical relations as the 

 determining factors. In favor of this view we may cite the anatomi- 

 cal evidence contributed by Lloyd 15 in the Rubiaceae, Longo 17 on 

 the Cucurbitaceae, and the observations of Capus 6 and of Gueguen, 11 

 and the results of the experimental studies of Miyoshi w , 2l and 

 others. Molisch 22 has shown that pollen-tubes are positively 

 chemotropic and some negatively aerotropic. Miyoshi sowed pol- 



len - 



ary 



and ovules of different degrees of development. Tne pollen-tubes 



