148 NECTARIES 



consequence secretion of water ceases, if the cells be killed by 

 painting the surface of the leaf with a solution of corrosive 

 sublimate or other poison. Hydathodes possessing water-pores 

 are, however, mainly passive in their action, the water being 

 forced out by hydrostatic pressure through the intercellular 

 spaces of the epithem, and not by active secretion on the part 

 of the living protoplasm. 



Insects are most commonly attracted to flowers by honey 

 formed in special structures termed nectaries. The honey is 

 produced by the active secretion of cells belonging either to the 

 modified epidermis (usually palisade- or papilla-like, the latter 

 in the Wood Anemone, Anemone nemorosa) or to the underlying 

 tissue. The nectary of the Hogweed or other common member 

 of Umbelliferae, forming the disc on the top of the ovary, is seen 

 in a cross-section to consist of a mass of small glandular cells 

 having the customary thin walls, abundant protoplasm, and 

 large nuclei, and covered by an epidermis containing numerous 

 stomata. The prominent cuticular ridges often exhibited by the 

 epidermis of these nectaries may serve to retain the secreted 

 fluid in situ. In the Buttercup the secreting tissue is similarly 

 composed of small cells, but there are no stomata, so that the 

 nectar only escapes by filtration through the outer membrane. 



Extrafloral nectaries on the vegetative organs occur in quite 

 a number of plants, e.g. on the under-surfaces of the stipules of 

 the Broad Bean (Vicia faba), on the leaf -bases of the Black Bind- 

 weed (Polygonum convolvulus), 'at the forks of the fronds of the 

 Bracken (Pteris aquilina, Fig. 74, A), and on the upper part of 

 the petioles of the Guelder Rose (Viburnum opulus). In the first- 

 named plant the nectary appears as a dark depression, which 

 when cut across is found to be composed of a palisade-like. layer 

 of secreting hairs, each consisting of an oblong head of several 

 cells, borne on a short stalk. In the Guelder Rose the nectaries 

 take the form of short cup-like projections, each supplied with 

 a vascular bundle, the actual secreting surface being situated in 

 the depression at the tip, and being similar in structure to that 

 of the floral nectary of the Hogweed. The structure of the 

 nectaries of the Bracken will be apparent from a reference to 

 Fig. 74, C. 



In all nectaries it is primarily the secretion of osmotic sub- 



