844 



ECOLOGY 



associated the more specialized estival or tropical insects with long probosces. The 

 attraction of pollinating insects is not the only advantage derived from floral nectar; 

 there has been previously noted the possibility that nectar associated with the sta- 

 mens may withdraw water from anthers, causing their dehiscence. It has been 

 suggested also that nectar may play some part in the maturation of fruit, and that it 

 may help to protect flowers from desiccation; the last-named role seems especially 

 evident in the case of flowers with water calyxes. 



Nectar is secreted by special structures, known as nectaries, and there 

 exist all gradations between those which are composed of undifferen- 

 tiated nectar-secreting tissue and those which are specialized glandular 

 hairs of complex structure. Usually they are 

 associated with the corolla, but they may be con- 

 nected with the stamens or with any other floral 

 part, even with the involucre (as in the poin- 

 settia. Euphorbia pulcherrima) , or they may 

 occur on vegetative organs, where they are called 

 exlrafloral nectaries (p. 858) ; in the poinsettia, 

 insects may get the abundant nectar without 

 pollinating the flowers, and in the case of the 

 extrafloral nectaries the visitors rarely are effi- 

 cient pollinating agents. The secreting regions 

 are composed of epidermal cells rich in cyto- 

 plasm; commonly they are long and narrow and 

 closely packed in palisade-Uke rows (fig. 1172). 

 Nectaries differ from other glands chiefly in 

 secreting sugar ; the process is not well under- 

 stood, although the presence of sugar outside the cell causes the with- 

 drawal of water from within and the consequent formation of a drop 

 of nectar, of which sixty to eighty-five per cent usually is water. 

 Sometimes the nectar forms in sufficient quantity to drip from the 

 secreting surface, and in some such cases it collects in protected pouches 

 or sacs, which usually are corolla structures known as spurs (fig. 1171). 

 In most cases the secretion of nectar occurs only at anthesis, though 

 it may continue for some time after pollination, as in the tulip and 

 the quince. The secretion of water, but not of sugar, is greater in 

 humid than in dry weather, quite as with hydathodes. Indeed, 

 there exist all gradations between nectaries and hydathodes. Espe- 

 cially interesting transitional forms are seen in certain tropical 

 flowers, whose glandular hairs secrete but little sugar, though exuding 



Fig. 1 1 72. — A longi- 

 tudinal section through a 

 floral nectary of the poin- 

 settia (^Euphorbia pulcher- 

 rtma), showing palisade- 

 like secretory cells (a) 

 which are rich in cyto- 

 plasm; highly magnified. 



