KIRKWOOD: POLLEN-TUBE IN CUCURBITACEAE 335 
for in this case they occur in the ovarian cavity. This has already 
been suggested by Lloyd" in discussing the pollen-tube in the 
Cucurbitaceae and Rubiaceae. 
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 Me/othria. 
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 Mcrampelts 
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 Melothria 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, 11,14). In theinterior 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,™ Zinger ** and some others have inter- 
preted the andotropic 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 ™ in the Rubiaceae, Longo” on 
the Cucurbitaceae, and the observations of Capus ° and of Guéguen," 
and the results of the experimental studies of Miyoshi”, and 
others. Molisch 2? has shown that pollen-tubes are positively 
chemotropic and some negatively aerotropic. Miyoshi sowed pol- 
len-grains on agar in which were imbedded pieces of stigma, wibead 
and ovules of different degrees of development. Tne pollen-tubes 
