146 VEGETABLE PHYSIOLOGY 



ascending sap ; this sap thickens as it ascends, in proportion as it dilutes and dissolves 

 the materials in the cells ; but to the motive force of the endosmose and capillary 

 action is added another not less powerful : this is the attraction exerted from above 

 by the buds, which draw up the food necessary to their development, and by the 

 already formed leaves, from the surface of which copious evaporation is carried on. 

 The empty spaces resulting from this evaporation and from the substance assimilated 

 by the buds, are filled by the sap in the parts immediately below ; these repair their 

 losses in their turn, and this action is continued from above down to the roots, for 

 which the soil is the reservoir. 



The buds are the first organs of the vegetable which awake in spring from their 

 winter torpor ; when they begin to swell, the resulting movement of the sap stimulates 

 the roots, which recommence their functions ; from this time the ascending current, 

 assisted by the endosmose, is established through the swollen tissues of the thickened 

 materials deposited the preceding year. Nevertheless, although it is the buds which 

 give the roots the signal to recommence their work, the work of the roots is carried 

 on independently of the influence of the buds ; for these remain closed long after the 

 sap has begun to rise with remarkable force and abundance. If at the period of 

 the spring sap an incision is made in a stem, a stream of sap flows from it, and the 

 proof that neither the buds nor the leaves are the cause of this phenomenon is that 

 it occurs just the same on a stem deprived of buds and leaves. An example of this 

 is seen in the tears of the vine, which flow from the stem when the plant is pruned, 

 and even when it is cut almost to the ground ; but as the buds lengthen, and as the 

 branches resulting from their elongation become covered with leaves, the suction of 

 the young branch and the evaporation from the surface of the leaves become active 

 forces, which join those of the endosmose and capillary action to assist the ascension 

 of the sap. 



When the branches are developed and consolidated, the movement of the sap 

 slackens, but without ceasing ; its only object now is to provide for the daily require- 

 ments of the plant, and to prepare materials for the vegetation of the following year. 

 When the spring rise of sap has taken place early, these materials are prepared before 

 autumn, and then the August sap is produced, which represents a second spring. 



In the autumn, the tissues, more and more solidified, dry up ; the leaves, of 

 which the canals become obstructed by a continual efflux of materials, cease to vege- 

 tate, and fall ; evaporation is thenceforth arrested, and with it the movement of the 

 sap ; and finally, life is suspended for several months. 



The ascent of the sap does not always take place in the same manner ; in spring 

 it rises across all the woody tissues ; in old branches, across the sap-wood only. Later, 

 most of the vessels are empty except of gases ; it is then by the cellular tissue that 

 the sap rises to support the vegetation. 



When the sap, laden with the materials that it has dissolved in its ascending 

 and diverging march, has reached the young branches, it penetrates their cortical 

 pith and the parenchyma of the leaves ; there it finds itself in contact with the air 

 which has penetrated by the stomata into the intercellular spaces ; then it under- 

 goes important modifications, and loses a large portion of its water, which evaporates 



