Kirkwood: Pollen-tube in Cucurbitaceae 333 



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 Micrampclis 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 Melothria, 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 Micrampclis. Both the epidermal 

 covering of the placenta and that of the ovary-wall lying opposite 

 (fig. 14) are alike filled with starch, but those on the placenta are 

 larger and more columnar in character. Here to a 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 Cacurbita 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 

 is along the conducting-tissue just described. In Melothria, Mic- 

 rampelis and Cyclanthera the tubes proceed down through the 



tyl 



The tubes pro- 



ceed upon the surface of the lobes wherever possible and not 

 through them as a rule. 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 



