802 PHYSIOLOGY. 



descend through the inner bark and cambium-layer, and seve- 

 ral facts seem to support the view. The formation of wood is 

 obviously from above downwards. When a ligature is tied 

 tightly round the bark of an Exogenous stem, or more especially 

 if a ring of bark be removed, no new wood is produced below 

 the ligature or ring, while there will be an increased development 

 above it, or roots will be produced there. Again, it is well 

 known, that by removing a ring of bark from a fruit tree, a 

 larger quantity of fruit may be temporarily obtained from that 

 tree, owing to the larger amount of nutritive matter which then 

 becomes available for the use of the reproductive organs (see p. 

 781). Another circumstance which appears to show the line of 

 descent of the nutritive matter, is the fact, that if the cortical parts 

 of the stems of a potato plant be peeled off, the formation of 

 under-ground tubers is prevented. Mr. Herbert Spencer, how- 

 ever, argues that the retrograde motion of the sap is through the 

 same channels — chiefly, as he believes, the vessels of the newest 

 wood — by which it passed up. He considers that this descent 

 takes place in response to a demand for liquid by the stem and 

 roots when evaporation from the leaves is at a standstill, as at 

 night. As far as the leaf-petioles are concerned, the back cur- 

 rent must be along much the same tissues as the upward flow ; 

 but probably the liber-cells of the petiole are the main channel, 

 and these are directly continuous with the inner bark of the 

 stem.* 



CHAPTER 3. 



SPECIAL PHENOMENA IN THE LIFE OF THE PLANT. 



1. Development of Heat by Plants. — As the various parts 

 of living plants are the seat of active chemical and other changes 

 during their development and in the performance of their dif- 

 ferent functions, we might conclude, that their temperature 

 would rarely or ever, under natural circumstances, correspond 

 with that of the atmosphere around them. 



We have already noticed, that during the germination of seeds, 

 a considerable development of heat takes place (p. 788). This 



* Mr. Spencer has described and figured (Linn. Soc. 

 Transactions, xxv.) cellular masses which he finds at the termi- 

 nation of the vascular system in the lower layer of parenchyma 

 in many leaves, and which he considers to be undoubtedly 

 absorbent organs by which the elaborated sap is abstracted 

 from the leaves ; his conclusions, however, require confirmation. 



