44 PLANT LIFE ON THE FARM. 
The word “sap,” then, though convenient, must not be 
used or conceived of as indicating the existence of a cur- 
rent absolutely fixed in its direction or uniform in its 
composition. In other words, it has buta remote analogy 
to the bléod, with which it isso often compared. There is 
an upward current of watery fluid, well marked in spring, 
there are downward and cross currents varying in direc- 
tion and intensity according to the requirements of the 
growing tissues and their conformation. These have 
only indirect connection with the main upward flow just 
referred to. 
CHAPTER ITI. 
GROWTH. 
Growth and extension.—Growth of cells.—Growing points.—Growth of 
roots, stems, and Jeaves.—Form as dependent on growth.—Move- 
meats dependent on growth.—Morvements of protoplasm,—Turges- 
cence.—Circumnutation of roots, stems, leaves.—Seedling plants, 
In considering the growth of. plants we have to distin- 
guish that growth which is mere extension of old material 
from that which is the result of the formation of new 
substance. We have an illustration of the first case in 
the earliest stages of germination of a seed, or in the 
sprouting of a potato in acellar. Growth may and does 
take place in such instances without any real increase of 
substance, or any augmented weight save what may be 
derived from water. The plant in this stage lives upon 
the resources stored up in its tissues, and will continue 
to do so until they are exhausted. But growth, in the 
sense of real increase of substance or of increased weight 
_ from the addition of new material, depends upon the 
‘amount of carbon assimilated, as already referred to 
under the heading of leaves. A plant with leaf-green or 
