170 THE PLANT CELL. 



which is as yet somewhat vindecided, and beyond the scope of an elemen- 

 tary text-book. It may, however, be mentioned, that the synthetic- 

 processes involve the interaction of substances known as aillidO-aCidS 



(asparagin), and a carbohydrate, together witii a sulphur-Gontaining' 



compound. The assimilation of nitrogen ia thus a process of ehemo- 

 SynthesiS as opposed to photosynthesis, and can proceed in the 

 absence of light. 



\A'itli regard to the ultimate fate of carbohydrate and nitro- 

 genous materials in the cell, it is important to remember that of 

 these essential food-substances, there are two main parts — viz., 

 that which is at once utilised by the cell-protoplasm, and known 

 as circulating proteid, or carbohydrate, and that which forms 

 stored or reserve food. As has been seen, the various enzymes 

 are constantly at ^^•ork converting reserve starch and proteid into 

 soluble substances, which can pass from cell to cell, from the parts 

 where they are manufactured or stored to remoter cells of a plant. 

 If a leaf in which starch has been actively formed duiing the 

 daytime be examined early, before the next day's assimilation 

 has started, it will be found that during the night-time all the 

 starch has been used up, in fact, has been converted into sugar, 

 which has been transjjorted to other parts as circulating food- 

 material. The same may be said of the asparagin (an amido- 

 acid), formed in the leaves : this substance quickly passes away 

 from the leaf-cells, and is, together with carbohydrates and 

 sulphur-compounds, constructed into proteid and protoplasm in 

 remoter parts. It has been shown that hydrocyanic acid can at 

 times be used for proteid construction in the place of an 

 amido-acid. 



iv.— The Cell-sap and the Mechanics of Sap-conduction. 



The cell-sap is a fluid which varies somewhat in composition 

 according to the part of a plant from which it is taken. Thus, 

 the watery sap which is present in the root-hairs of a root, and 

 is conducted upwards by the woody portions of root or stem, 

 contains far less solid matters in solution than the sap of the 

 elaborating cells of the leaf, or of the downward-conducting 

 elements of the phloem. Nevertheless, from a general point of 

 view, it may be said that the cell-sap is made up of the follow- 

 ing substances : — 



a. Water.— In some cases 08 per cent, of the sap is composed of water. 



h. Mineral matters in solution — viz., salts of sodium, potassium, 



