THE MOVEMENT OF WATER IN PLANTS. IOI 



have been observed of a similar process in other parts of the 

 plant. The most familiar instance is the excretion of nectar 

 by nectaries. The structure of a nectary is essentially similar 

 to that of a water-gland (see fig 19, p. 91), but its properties 

 are very different. We have seen that the excretion of liquid 

 by a water-gland is entirely dependent upon the root-pressure ; 

 when the organ bearing the water-gland is separated from the 

 rest of the plant, the secretion at once ceases. It is not so 

 with a nectary. If a flower containing nectaries, that of 

 Fritillaria imperialis for example, be cut off, and the drops of 

 nectar be removed by means of blotting-paper, it will be found 

 after a time that large drops have been excreted. The cells of 

 the nectary, like those of the parenchymatous tissue of the root, 

 have the power of setting up within themselves so great a 

 hydrostatic pressure as to force out by filtration the fluid 

 which they contain. This is true also with regard to the 

 pitchers of Nepenthes. But it must not be supposed that 

 this property is confined to special organs ; it is probable, as 

 suggested in a previous lecture (p. 45), that it is possessed 

 very generally by the parenchymatous cells of the plant, and 

 that it plays an important part in the distribution of substances 

 which, though soluble in water, do nof readily diffuse. Sachs 

 has observed, for instance, that if pieces (6 10 ctm. long) of 

 the young haulms of various Grasses be placed with one cut 

 end in damp sand, and be kept in a moist atmosphere, drops 

 of water will exude at the other end. Similar observations 

 have been made by Pitra. The excretion of liquid in these 

 cases is doubtless due to the fact that the parenchymatous 

 cells become tensely filled with water ; a considerable hydro- 

 static pressure is set up in the cells, and the result is the 

 escape of water by filtration, probably into the vessels. This 

 may be readily observed whenever any turgid succulent tissue 

 is cut across ; drops of water are formed at the cut surface, 

 which are derived partly from the cells which have been cut 

 open, but also, probably, from uninjured cells which lose 

 water by filtration in consequence of the alteration of the con- 

 ditions of tension in the tissue which has been induced by 

 section. 



