KIRKWOOD: POLLEN-TUBE IN CUCURBITACEAE 3 an 1 
that the ovules are arranged in one series instead of several on 
each flank of the three longitudinally-running placental lobes. 
Each of the placental lobes is covered with a single layer of cells, 
abounding in starch and possessed of denser plasmatic contents 
than those underneath. This layer also lines the narrow stylar 
canal, and covers the funicles of the ovules. 
In Micrampelis the structure of the ovary is more complex. 
Here the normal ovary contains four ovules with their micropyles 
directed toward the pedicel. These are borne one on each flank of 
the two inwardly-growing placental lobes. In this case, however, 
the placental ridges become partially fused and grow downward 
toward the base of the ovary. In this genus as in Me/othria, the 
cells which line the stylar canal and cover the placental lobes are 
filled with starch and denser protoplasm. Starch is also found in 
the underlying cells to a greater degree than in Melothria, but it 
is most abundant near the stylar canal and ovarian cavity. 
Cyclanthera presents a condition in which there is one placental 
ridge in the Ovary with a row of ovules on each flank. Here too 
the covering of the placenta and the lining of the stylar canal is 
similar to that of Melothria and Micrampelis. Both the epidermal 
covering of the placenta and that of the ovary-wall lying opposite 
(Fic. 14) are alike filled with starch, but those on the placenta are 
larger and more columnar in character. Here toa greater degree 
‘than occurs in the other cases the tissue beneath the epidermis is 
Supplied with starch. Quite different from this is the condition 
found in Cucurbita Pepo, in which the conducting-tissue is devoid 
of starch. In Cucurbita the conducting-tissue consists of several 
layers of cells formed by the periclinal division of the epidermal 
cells covering the placentae. 
The course of the pollen-tube in all cases under consideration 
's along the conducting-tissue just described. In Melothria, Mic- 
“ampelis and Cyclanthera the tubes proceed down through the 
Stylar canal and over the surface of the placentae. The tubes pro- 
‘eed upon the surface of the lobes wherever possible and not 
through them asarule. In the restricted space of the style, where 
the canal offers insufficient space to accommodate many tubes, they 
traverse the surrounding tissues and break them down to a con- 
siderable extent. In such conditions, and elsewhere, where the 
