Nectaries on Leaves. 137 



mwrn tinus, and V. opulus, and GlerodeTidron fragrans, 

 in all of which the leaves undoubtedly secrete nectar. 



In some of these, again, the secretion is produced by 

 special groups of epidermal cells, placed on the under 

 side of the leaf or stipules, and transformed into glan- 

 dular tissue, as is the case, for instance, in Clerodendron, 

 Prunus laurocerams, and the above-mentioned species 

 of Vicia ; while in others the nectar is secreted by 

 disk-shaped or knob-headed trichomes, which are found 

 springing from the surface of the leaf or of the petiole, 

 as is the case in Catalpa, Viburnum opulus, and Prunus 

 armeniaca. 



In Impatiens trioornis the two small stipules of 

 each leaf are entirely transformed into nectaries. One 

 of these is very small and stunted ; the other, on the 

 contrary, forms a fleshy disk, slightly convex above, 

 and below shaped into a semi-globular protuberance, 

 which is partly adherent to the base of the petiole, 

 partly to the epidermis of the stem, and lies trans- 

 versely in front of the axil from which the flower-stalk 

 sprmgs. (See Plate III. fig. 117; also fig. 118, which 

 represents a transverse section of such a transformed 

 stipule.) The nectar secreted by the tissue of this 

 fleshy disk collects in the form of a drop on the apex of 

 the semi-globular breast-shaped swelling on the under 

 side of the disk. Any insects that creep along the 

 stem must, if they would get at the flower, of necessity 

 pass over this disk with its drop of nectar; thus what 



