780 



PHYSIOLOGY. 



•when the plants are most abundantly covered with leaves, and 

 when solar light is most intense, we ought not to transplant at 

 such periods, because as it is almost impossible to do so without 

 some injury to the extremities of the roots (see p. 730), the 

 amount of fluid absorbed cannot compensate for the loss by 

 transpiration, and hence the plants will languish, or die, accord- 

 ing to circumstances. By transplanting in autumn or spring, 

 we do not expose the plants to such unfavourable influences, as 

 the light is then less intense, and there are no leaves upon them. 

 (For further particulars on Absorption, see Absorption of Hoots, 

 p. 730.) 



2, Distribution of Fluid Matters through the Plant, and their 

 Alteration in the Leaves. — The fluid matter thus absorbed by 

 the roots is carried upwards by their tissues (fig. 1117) to the 

 stem, and thence through it to the leaves, &c., as indicated by 

 the arrows in the figure, to be aerated and elaborated, after 

 which it is returned to the stem and descends by the inner bark 

 and cambium layer (Jig. 1117) of Dicotyledons towards the 

 roots from which it started; and 

 by means of the medullary rays 

 and the general permeability of 

 the tissues of which the plants are 

 composed, it becomes gradually 

 distributed to the different parts 

 of the plant where new tissues 

 are being formed, and where se- 

 cretions are to be deposited. This 

 general distribution of the fluid 

 matters through the plant is com- 

 monly termed the circulation of 

 the sap. The fluid as it ascends 

 is called the Ascending or Crude 

 Sap, and as it descends, the De- 

 scending or Elaborated Sap. Al- 

 though the term Circulation is thus 

 commonly applied to this move- 

 ment of the saj), it must be borne 

 in mind, that the process bears no 

 analogy to the circulation of the 

 blood in animals; for plants have 

 no heart or any organ of an ana- 

 logous nature to propel their 

 fluid matters, nor any series of 

 vessels in which a flow thus produced takes place. The circula- 

 tion of tiic sap of plants is simply a distribution of fluids, and 

 the essential and direct cause is, endosmotic action. 



Ascent (f the Sap. — The sap in its ascent to the leaves, &c., 

 passes principally through the young unincrusted ^wood-cells 



Fiff. 1117. Diagrammatic section of 

 the stem of a Dicotyledon, showing 

 the distribution or circulation of 

 the sap. The direction is indicated 

 by the arrows, a, a. Roots, by 

 wliich the fluid matters arc ab- 

 sorbed, h, h. The tissues by which 

 tlicy ascend to the leaves, c,r. r/, d. 

 Outer portions of Htem and bark 

 ■where the descent takes place, c. 

 Section of a branch. After Bal- 

 four. 



