THE EXCRETION OT \ECTJK 283 



sugar or other substances is exhibited by certain plants only, and may be 

 modified in the progress of development as well as by the external conditions. 

 A single thorough rinsing with water may suffice to stop completely the 

 excretion of nectar from the large nectaries at the base of the perianth of 

 fully opened Fritillaria flowers, whereas in quite young flowers the washing 

 with water must be repeated two to four times, and in Primus lanrocerasus 

 five to six times in the course of one to a few days to produce the same 

 result. It appears that in certain cases fresh sugar is continually excreted, 

 for Schimper 1 found that the excretion of nectar from the petiolar nectaries 

 of Cassia neglecta took place continually, although they were daily rinsed 

 with water for eight days. Such a plant may lose a large amount of sugar 

 if the nectar is continually washed away by rain or removed by animals, 

 but this is simply in correspondence with the general law that an increased 

 demand causes an increased production. 



The secretion and excretion of an osmotically active substance (sugar) 

 is the only vital process which takes part in the formation of nectar. This 

 sugar is usually secreted by the gland-cells of the nectary, but in many 

 cases a certain amount of sugar is produced by a metamorphosis of the 

 cell-wall, possibly clue to some vital action exerted by the plasma within. 

 This latter process usually seems to occur only once, when the secretory 

 activity first commences ; and since all nectaries can apparently excrete 

 sugar, it seems probable that this metamorphosis of the cell-wall is not 

 for the purpose of providing extracellular sugar, but is of primary im- 

 portance in rendering the excretion of nectar more easy, by increasing 

 the permeability of the partially metamorphosed cuticular walls. The 

 cuticle may, however, be readily permeable at the outset (Sects. 21 and 27). 

 and hence it is not surprising to find that in other nectaries and water-glands 

 it may persist unaltered. 



The general principles regulating the excretion of sugar by nectaries 

 have already been discussed (Chap. IV). It is, however, not precisely 

 known how the glandular cells are able to excrete sugar, though when 

 once this occurs, the subsequent processes are comparatively simple and 

 easy to understand. Other points also are not quite clear ; it has yet 

 to be determined how the protoplasts of the gland-cells manage to avoid 

 being injured by the relatively enormous osmotic forces which act upon 

 them when the nectar becomes highly concentrated (see Sect. 23), and 

 moreover it is not easy to see why the sugar is retained in the nectary 

 instead of being sucked backwards into the tissues. There must be some 

 hindrance or other to the backward filtration of sugar through the gland- 

 cells ; and perhaps the continual excretion of sugar may prevent any 

 backward flow, for when this excretion ceases and the nectar dries up, 



1 Schimper, Pflanzen u. Ameisen, 1888, p. 7.' ; also Bib-en, Honigthnu, i!-'yi. p. 32. 



