508 SECRETORY AND EXCRETORY SYSTEMS 



walled parenchyma is also more or less glandular in character. The 

 superficial cell-layer is often furnished with stomata, which, according 

 to Behrens, often act as nectar-valves and sometimes with arrange- 

 ments for keeping the secretion in contact with the secreting surface. 

 In many Umbelliferae, for example {Pastinaca sativa, Heracleum 

 Sphonth/1 in in , Daucus Carota), the outer walls of the nectar-secreting cells 

 are provided with cuticular ridges, arranged in an intricate curvilinear 

 pattern, which help to retain the nectar by adhesion. The " septal 

 nectaries," which occur among the Liliifloiiae and Scitamineae, 

 belong to the category of internal glands ; according to Grassmann, 

 they arise by the partial non-coalescence of the margins of adjacent 

 carpels. When fully developed, they consist of branched or un- 

 branched canals and crevices in the substance of the pericarp, lined by 

 papillose or palisade-shaped secretory elements. Special outlets are 

 provided for the escape of the nectar. 



With regard to the mode of secretion of nectar, it is necessary to 

 discriminate between two distinct processes namely, the exudation of 

 some osmotically active substance, and the subsequent escape of water, the 

 actual nectar being formed by the interaction of these two substances. 

 The osmotically active material may be liberated by one of two 

 methods : either certain layers of the superficial walls undergo an 

 appropriate chemical modification ; or else the compounds that initiate 

 the "osmotic suction" exude from the interior of the secreting cells. 

 That the escape of water from nectaries depends upon " osmotic 

 suction," has been proved experimentally by Wilson. The flow of 

 liquid ceases immediately, if the osmotically active substances are 

 removed by washing with water, and starts at once again if a minute 

 fragment of moistened sugar is placed on the surface of the nectary. 

 Nectar that is, as already explained, the actual secretion diluted with 

 water always contains a large amount of sugar ; according to Bonnier, 

 gum, dextrine, mannite and certain compounds of nitrogen and 

 phosphorus may also be present in small quantities ; the same author 

 states, that the water content of nectar varies between 60 and 85 

 per cent. 



A great many plants are furnished with so-called extra-nuptial or 

 extra-floral nectaries, which may occur within the floral region, on sepals, 

 bracts, etc., or on purely vegetative parts, such as petioles and stipules. 

 According to Belt, Delpino and others, these organs serve to attract 

 protective ants, or, as in the case of Nepenthes and Sarracenia, to lure 

 insect victims to their doom. Extra-nuptial nectaries exhibit con- 

 siderable diversity as regards their histological features. Some of 

 them closely resemble ordinary floral nectaries ; others are mainly or 

 exclusively composed of thin-walled palisade-like secretory elements, 



