44 



HUTCHINSON'S POPULAR BOTANY 



treated with iodine, which cellulose will not. Their 

 formation may be thus described. The cells con- 

 taining chlorophyll, which are always near the 

 surface of the plant, absorb carbonic acid gas (C0 2 ) 

 from the atmosphere or water (the latter in the case 

 of submerged plants), and this gaseous compound 

 reacts with water (H 2 O) in the chlorophyll corpuscles 

 under the action of light. The first organic product 

 as a result of this process is, in most plants, glucose 

 (C 6 H 12 6 ), or some other form of sugar. The sugar 

 has to be diffused along certain delicate cells * of the 

 plant, and as the process of diffusion is too slow to 

 keep pace with the process of construction, another 



agency is brought into play - . The chlor p lasts > in 



short, have the power of converting sugar into starch 

 a power (we quote from Dr. Reynolds Green) which " is quite independent 

 of the colouring matter, being shared by other quite colourless plastids [the 

 leukoplasts already mentioned], which occur in other parts of the plant. 

 The transformation is apparently a process of secretion. Part of the 

 sugar consequently gives rise to numerous minute grains of starch, which 

 the plastid forms within itself, and deposits in its own substance. This 

 formation of a temporary store not only relieves the over-saturation of 

 the sap in the cell, but supplies the need of the protoplasm when the 

 formation of sugar from carbon dioxide and water is interrupted by the 

 failure of the daylight." It has been estimated that one hundred square 

 yards of green leaves can during five hours of sunlight manufacture one 

 pound of starch. 



In this way, then, do green plants assimilate the carbon which they take 

 into their cells by absorption ; and as carbon usually 

 forms one-half of the dried plant by weight, the 

 statement will not appear extraordinary that starch 

 (or its physiological equivalent) is really the raw 

 material from which all the other organic substances 

 of the plant are elaborated. 



Starch-grains are found in almost all plants, in 

 every part, but particularly in the roots, tubers, 

 seeds, and fruits, where they are stored up as reserve 

 food material : in fact, the}^ supply the young plant 

 with food till it is in a condition to feed itself. 



RThe roots of the Tapioca-plant (Jatropha manihot) 

 yield about 13| per cent, of this important sub- 

 stance ; the tubers of the Potato-plant (Solanum 



FIG. 68. RASPBERRY tuberosum) nearly twice that proportion (figs. 63, 65) ; 

 (Rubus id<Bus). * The bast I tissue (vide Chapter III.). 



