EXTRA-FLORAL NECTARIES 



509 



representing modified epidermal cells, or of groups of trichomes, which 

 are most frequently clavate or shield-shaped in form, though other 

 types also occur. In Vicia sepium, for example, and in other species 

 of the same genus, every stipule bears, on its upper surface, a nectary 

 composed of densely-crowded club-shaped hairs (Fig. 205). Each hair 

 is made up of a basal cell, a short stalk-cell, and three or four 

 glandular cells, which contain abundant protoplasm and numerous 

 tannin-vesicles. Fehling's test gives a heavy precipitate of cuprous 



Fig. 205. 



Extra-floral nectaries of Vicia sepium. A. T.S. through a stipule, traversing the 

 nectary. B. Margin of the nectary, more highly magnified. 



oxide in the glandular cells, a circumstance which indicates the presence 

 of a considerable amount of sugar. These clavate secretory trichomes 

 are accompanied by a smaller number of much longer pointed hairs 

 (three to six times as long as the club-shaped hairs), which are possibly 

 designed to retain the nectar by capillary action. The extra-nuptial 

 nectaries of certain species of Dioscorea have been fully investigated by 

 Correns. These are sunken, glandular structures which differ in shape 

 according to their location ; they are ellipsoidal on the (abaxial) leaf 

 surfaces, but spindle-shaped on the petioles and internodes. Each 

 nectary arises from a single protodermal mother- cell. The superficial 

 layer of the fully-developed gland has suberised walls, and thus bears a 

 certain resemblance to an endodermis. 



Even more remarkable nectaries occur, according to Zimmermann, 

 on the base of the petiole and also on the lamina, in the genus Fagraea. 

 Here the palisade-shaped (columnar) secretory cells enclose a cavity 

 whicli ramifies more or less extensively in the substance of the leaf, and 

 opens to the outside by a passage directed at right angles to the surface 



