284 THE MOVEMENTS OF WATER 



no sugar is left behind. This may be observed on many nectaries which 

 have been functionally active for a long time (Wilson, I.e., p. 13), whereas 

 others still remain filled if the flower falls off before its tissues are dead. 

 Even when the sugar is ultimately reabsorbed and used in metabolism, 

 there is no reason to suppose, as Bonnier l does, that the nectar is to be 

 regarded mainly as a store of reserve food-material, for a more wasteful 

 and dangerous mode of storing reserve food could hardly be conceived. 

 Besides, it is well known that by means of this food the attraction of insects 

 is secured, which either ensure cross fertilization, or protect the plant from 

 foes, or are of use to it in other ways. 



The works here quoted 2 give a full account of the form, structure, and distri- 

 bution of both floral and extra-floral nectaries, and of the glandular sugar-secreting 

 cells which form an essential part of each nectary. The nature and structure of an 

 active nectary must in every case be such that the conditions already laid down can 

 be fulfilled. The glandular secreting cells are often small and possess dense contents ; 

 they are usually superficial in position, but may be more deeply situated with ducts 

 leading to the exterior. In a compound nectary of this type the glandular areas 

 may abut so closely upon one another as to form a continuous layer, although the 

 dutts may remain distinct. It is only in certain cases that the cuticle is thrown 

 off more or less completely, while it is easy to understand that all the starch the 

 secreting cells contain may partially or entirely disappear when sugar is actively 

 excreted. A supply of water and of food-material is necessary in order that a con- 

 tinuation of the secretory activity of the nectary may be possible. 



Historical. No distinction was made between the excretion of water by active 

 exudation and by nectaries until Pfeffer's Osmotische Untersuchungen appeared 

 (1877, p. 232). The ideas there put forward have been confirmed by Wilson's 

 researches. The doubts thrown by certain authors upon the occurrence of any 

 plasmolytic excretion of water can only be maintained by ignoring the physical and 

 physiological principles involved, and by neglecting to distinguish between the 

 excretion of the sugar and of the water, that is, between the cause and the result 3 . 



Composition". In addition to sugar, only small quantities of other sub- 

 stances are present in the nectar, but their existence is indicated by the odour, taste, 

 or acid reaction of the nectar from certain plants. Minute traces of nitrogenous 

 substances may be present, although von Planta could not detect any in the nectars 

 he examined. The sugar may occur in the form of dextrose, laevulose, saccharose, 

 or even mannitose (Fraxinus> Sambucus}> while dextrin or other carbohydrates 



1 Bonnier, Ann. d. sci. nat., 1878, vi. se"r., T. vni, p. 199. 



2 De Bary, Anat., 1877, p. 93; Kerner, Pflanzenleben, 1891, Bd. II, p. 168 ; Reinke, Jahrb. f. 

 wiss. Bot., 1876, Bd. x, p. 119; Bonnier, Ann. d. sci. nat, 1878, vi. se"r., T. vni, p. i ; Poulsen, 

 Bot. Centralbl., 1881, Bd. vi, p. 7; Grassmann, Flora, 1884, p. 13; Stadler, Beitrage z. Kennt. d. 

 Nectarien, 1886 ; Schimper, 1. c. ; Acton, Annals of Botany, 1888-9, Vol. II, p. 53 ; Wettstein, Bot. 

 Jahresb., 1888, p. 528, &c. ; Schniewind-Thies, Beitrage z. Kennt. d. Septalnectarien, 1897. 



3 Biisgen, Honigthau, 1891, p. 31. Cf. Pfeffer, Energetik, 1892, p. 267, and Sects. 46 and 47. 



1 See von Planta, Zeitschr. f. physiol. Chemie, 1886, Bd. x, p. 227 ; Bonnier, 1. c., p. 84, and the 

 literature quoted therein. 



