THE EXCRETION OF NECTAR 285 



which form mucilaginous solutions may also be present, as in the nectar from the 

 flower of Pingitiiiila alpina, in which, according to Stadler (I.e., p. 73\ no actual 

 sugar is present. This observation however requires confirmation. No cane-sugar 

 is present in certain nectars (for instance that of Proiea nielli/era, according to 

 von Planta), whereas in other cases most of the sugar is present in this form. 

 In the nectar from the flowers of Hoy a carnosa, 87-4 per cent, of the sugar is 

 cane-sugar. According to Bonnier, the maximum amount of cane-sugar is found 

 when the secretion is most active, so that saccharose apparently is in part inverted 

 after it has been excreted, and hence it arises that the greater part of the sugar 

 present in the nectar may be in the form of dextrose. In any case, however, 

 cane-sugar is nearly always found in the nectar, as Braconnot showed in 1841 J ; and 

 though it does not follow that this is the only form of sugar which can be excreted, 

 it is certain that saccharose does actually diosmose as such through the protoplast 

 (Sect. 1 6). In the works already mentioned, data are given concerning the varying 

 amount of water present in the nectar. 



The influence of tJie external conditions. As is often the case, the 

 secretion of nectar commences at a certain stage of development, which differs 

 in different plants, and ceases after a longer or shorter period of activity 2 . 

 The general vegetative conditions, as might be expected, also exercise 

 a more or less marked influence upon the excretion of nectar, which is 

 diminished in starved plants, owing to all the available food being prefer- 

 ably employed in internal metabolism. Apparently this was the reason 

 why Schimper found that the functional activity of the nectaries of Cassia 

 neglccta ceased in a few days when the plants were kept in darkness, 

 or in an atmosphere deprived of carbon dioxide, whereas the secretion 

 continued when the leaf was exposed to light and the nectaries only were 

 kept in darkness 3 . 



In certain cases light may also exercise an obscure but apparently 

 direct effect as well. Wilson 4 found that illumination causes the excretion 

 of nectar to commence in the stipular nectaries of l^icia faba, even in 

 an atmosphere free from carbon dioxide, whereas in darkness the excretory 

 activity is not developed, or even ceases if it has begun, although a plentiful 

 supply of food may be available. Similarly, it can hardly be due to the want 

 of food that the floral nectaries of ErantJiis hiemalis do not secrete when 



1 Cf. also E. Schnlze uncl Frankfurt, Zeitschr. f physiol. Chemie, 1895, Bd. XX, p. 532. [P. Knuth 

 (Uber den Nachweis von Nektarien auf chemischem Wege, Bot. Centralbl., Bd. i,xxvi, 1898, p. 76) 

 uses the precipitation of indigo from orthonitrophenyipropiolic acid in the presence of grape sugar as 

 a test for the nectariferous character of the gland-cells of supposed nectaries, but it is hardly possible 

 that this method can apply to all cases, for a reducing sugar need not necessarily be present.] 



2 For actual data, see Bonnier. 1. c., p. 192 ; Schimper, 1. c., p. 72. 



3 Schimper, 1. c., p. 75. On the influence of the soil, see Bonnier, Bot. Centralbl., Beihefte, 1894, 

 Bd. iv, p. 419. 



4 Wilson, 1. c., p. 12. The dependence of the secretion on light in this plant and in the flowers 

 of Lobelia erintis was first discovered by Darwin (Cross and SMf-fertilization. 1877. ? 3^ 



