44 PLANT LIFE OK 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 but a remote analogy 

 to the blood, with which it is so 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 III. 

 GROWTH. 



Growth and extension. Growth of cells. Growing points. Growth of 

 roots, stems, and leaves. Form as dependent on growth. Move- 

 ments dependent on growth. Movements 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 a cellar. 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 



