786 PHYSIOLOGY. 



generally of the stems of Monocotyledons and Acrogens. It then 

 descends in the stems of the several kinds of plants towards the 

 root, and in its course affords materials for the development of 

 new tissues and the production of flowers and fruit, and at the 

 same time deposits its various secretions. Hoffman in his ex- 

 periments upon Ferns, however, could not find any path by 

 which the elaborated juices descended in the stem. 



In Dicotyledons, the elaborated sap is commonly said to 

 descend through the internal bark and cambium-layer towards 

 the root, and to be transmitted laterally inwards by the medullary 

 rays. (See p. 734, and Functions of Laticiferous vessels, p. 727.) 

 The causes which lead to this descent of the sap are ahogether 

 unexplained, indeed, this descent is totally denied by some 

 physiologists ; but that a descent of assimilated nutriment does 

 take place is beyond doubt, for otherwise, how could we ex- 

 plain the fact, — that when a ligature is tied tightly round the 

 bark of a 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. Again, it is Avell 

 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. 

 Another circumstance which also clearly proves to my mind the 

 descent of the nutritive matter, is the fact, that if you peel otF the 

 cortical parts of the stems of a potato plant, the formation of 

 underground tubers is prevented. We may conclude therefore, 

 .that although we cannot explain the cause or causes which lead 

 to the descent of the elaborated sap, the fact of that descent is 

 undoubted. 



CHAPTER 3. 



SPECIAL PHENOMENA IN THE LIFE OF THE PLANT. 



1. DEVELOPaiENT OF Heat BY PLANTS. — As thc various organs 

 and parts of living plants are thc scat of active chemical and 

 other changes during their development, and in the performance 

 of their different functions, we might conclude, that their tem- 

 perature would rarely or ever, under natural circumstances, cor- 

 respond 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. 770). 

 This is more especially evident, when a number of seeds ger- 

 minate together, as in the process of malting. The development 



