PROPULSION OF SAP AND ITS VARIOUS EFFECTS 405 



loss of water by transpiration is greater than the supply, a 

 negative pressure will be observed. 



The ascent of sap primarily due to cellular activity, may 

 be secondarily aided by evaporation from the leaves, and the 

 osmotic action of the concentrated cell sap in the leaves. 



Owing to the distribution of unequally active cells, an 

 irregular variation of pressure is induced in the stem. 



The excitatory movement may be transmitted to a dis- 

 tance by conduction, or there may be conduction by ' relays.' 

 An isolated mass of highly excitable tissue may thus be 

 excited de novo. 



The excretion of water and of nectar are phenomena of 

 cellular activity, analogous to that which brings about the 

 ascent of sap. 



The translocation of food-material is also probably due, 

 at least in part, to excitatory reaction. 



The internal activity of the plant, causing increase of 

 turgidity, may be detected mechanically by that erection of 

 the leaf which is characteristic of the positive turgidity- 

 variation. 



Any increase of internal activity is exhibited in dorsi- 

 ventral organs, such as the petioles of Mimosa, Biophytum^ 

 and Artocarpus, by the erection of the leaf. Thus, when the 

 internal energy of the plant is increased by a rise of tem- 

 perature, the leaves become erected. Conversely, under the 

 action of cold, on account of the diminution of the latent 

 energy, the opposite effect, or droop, is induced. This 

 explains the drooping of various leaves during frost, and 

 their subsequent erection when brought into a warmer atmo- 

 sphere. 



This internal energy is also an important factor in 

 bringing about the recovery of an organ from the effect of 

 external local stimulus. The effect of external local stimulus 

 in causing the diminution of turgidity of an organ is thus 

 antagonised by the internal activity, which causes an increase 

 of turgidity. 



