THE ANGIOSPERMAE 1251 



a special bundle or bundles which may branch throughout the secretory 

 tissue if this is massive. The surface layer may have the columnar " epi- 

 them " structure or it may be undifferentiated. Frequently there is a 

 cuticularized epidermis, in which case the excretion of nectar takes place 

 either through stomata, which resemble water-stomata in being permanently 

 open, or through cracks in the cuticular covering. 



The quantity of nectar excreted is likewise very variable. There may 

 be no more than a surface film or there may be several cubic centimetres 

 collected in the spurs of some flowers. Where excretion is copious it may 

 drip from the nectary, as in Phygeliiis capensis or in the Orchid, Coryanthes, 

 where as much as 30 c.cs. may be produced. 



A mixture of sugars is present, fructose, glucose and sucrose being of 

 regular occurrence. The total sugar concentration varies greatly and is 

 roughly inverse to the volume of nectar excreted. Rough quantitative 

 measures show that the concentrations of fructose and glucose are closely 

 correlated but the concentration of sucrose shows an inverse relation to that 

 of the other two sugars. In some species three other sugars, maltose, 

 raffinose and melibiose, may be present in small quantities, though they 

 tend to be irregular. The last named is non-nutritive for bees but it attracts 

 them and in years of scarcity they have been known to collect it extensively 

 from honey-dew on leaves. 



The relation of nectar concentration to bee-visits will be dealt with in the 

 section on Pollination (see p. 1258). 



A curious relation between nectary excretion and the dehiscence of the 

 anthers has been suggested by Burck. It can be shown that, when ripe, 

 the anthers of many flowers lose remarkable amounts of water, not, as he 

 showed, by transpiration but by withdrawal of water internally to other 

 tissues. This, he suggests, is due to the nectaries, then beginning excretion, 

 which has the double advantage of attracting insect visitors at the right time 

 and of creating the conditions of hydrostatic tension in the anther wall which 

 lead to dehiscence. 



Porsch has pointed out an interesting systematic relationship in the 

 distribution of nectaries. The more primitive families of the Dicotyledons, 

 the twenty-three composing Wettstein's Polycarpicae, rely chiefly on the 

 perianth and the androecium for nectar production and seldom have toral 

 nectaries, which, on the other hand, are characteristic of Sympetalae. The 

 Monocotyledons agree with the Polycarpicae in this respect and display 

 close resemblances to them, even in details, another feature connecting the 

 Monocotyledons with the lower Dicotyledons. 



Lastly we mav mention the existence of false or substitute nectaries, that 

 is soft and juicy tissues, such as the cushion surrounding the base of the style 

 in Leucojum or juicy hairs like those on the stamens of AnagaUis and Trade- 

 scantia, which can be sucked or eaten by insects. Even the pulpy and dis- 

 coloured old petals in short-lived flowers like those of Eremiirus or Calan- 

 drinia, which have passed their anthesis, may attract juice-sucking insects 

 who may, by their visits, bring pollen to the still receptive stigmas. 



