82 BOTANY FOR BEGINNERS CHAP. 



The Cell-Sap is the watery fluid which saturates the proto- 

 plasm and the cell-wall, and also occupies the vacuoles ; it con- 

 sists of water which holds in solution a number of organic and 

 inorganic substances. The substances in solution are either on 

 their way to be built up into protoplasm, or have themselves 

 been formed by previously existing protoplasm. The organic 

 substances present in cell-sap are sugar, organic acids, proteids, 

 and in many cells colouring matter. The inorganic substances 

 are chlorides and sulphates of potassium and sodium. Solid 

 bodies, in addition to these dissolved substances, may also be 

 present in the vacuole, e.g., starch grains, aleurone grains, and 

 raphides or needle-like crystals of calcium oxalate. 



Chloroplasts. In the cells building up the green parts of 

 plants a green colouring matter is present called chlorophyll. 

 In all the higher plants the chlorophyll is found in the form of 

 granules known by various names, as chlorophyll grains, chloro- 

 phyll corpuscles, or chloroplasts. A chloroplast is a small 

 mass of protcplasm saturated with chlorophyll. This is shown 

 to be the case when a cell which contains chloroplasts is treated 

 with alcohol. The chlorophyll is dissolved out, and colourless 

 grains are left behind 7~these are called leucoplasts. 



It is only in those cells which are exposed to light that 

 chlorophyll is developed. The conditions necessary for the 

 development of chlorophyll, are : 



(a) a certain temperature, a few degrees above the freezing 

 point ; 



(b] light ; any light will do if it is only intense enough ; 



(t) a small quantity of iron in the food of the plant. The 

 necessity of iron for the development of chlorophyll is very 

 interesting, for no iron is found in the chlorophyll itself. The 

 iron is probably necessary in the chemical changes which result 

 in the formation of chlorophyll. 



From what has been said about light being necessary for the 

 formation of chlorophyll, it will be understood why it is found 

 only in the surface cells. The important function of chlorophyll, 

 which can only be exercised in the presence of light, is to absorb 

 the carbon dioxide in the atmosphere, and to split it up into 

 carbon and oxygen. The oxygen is returned to the air, but the 

 carbon combines with the elements of water to form sugar 



