724 PLANT RESPONSE 



of water from the plant is in any way arrested, a positive 

 pressure is produced, owing to its excessive accumulation. 

 Similarly, when loss is greater than supply, the pressure will 

 be negative. The ascent of sap, primarily due to cellular 

 activity, may be secondarily aided by evaporation from the 

 leaves, and by the osmotic action of the concentrated cell sap 

 there. Owing to the distribution of unequally active cells, an 

 irregular variation of pressure may be induced in the stem. 

 The excitatory movement may be transmitted to a distance 

 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 phe- 

 nomena 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, Biophy- 

 tu7n, and Artocarptis, by the erection of the leaf. Thus, when 

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

 temperature, 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 ex- 

 plains the drooping of various leaves during frost, and their 

 subsequent erection, when brought into a warmer atmosphere. 



