122 BOTANY FOR BEGINNERS ' CHAP. 



freezing point. Heat is as necessary here as in all other vital 

 processes. 



If a beam of white light is passed through a prism it is bent out of its 

 course and split up into a number of colours. These colours are red, 

 orange, yellow, green, blue, indigo, and violet. This experiment 

 shows that white light is built up of several primary colours. The 

 constituents of white light which the chloroplasts use for assimilation 

 are just the reverse to those used in chemical processes. The rays of 

 the spectrum, as the decomposed light is called, which act on the 

 sensitive plate of the photographer, or decompose silver salts, are those 

 from the violet end ; but the rays which are the most active in assimi- 

 lation are the red, orange, and yellow. If a plant is grown under 

 such conditions that only the red, orange, and yellow rays can reach it, 

 the assimilation is nearly as active as in white light for 90 per cent, of 

 starch will be formed against 100 per cent, in white light. On the 

 other hand, if a plant is grown so that only the violet and blue rays 

 can pass through, assimilation falls very low, to from 5 to 7 per cent, 

 only of the 100 in white light. 



The changes by which carbon dioxide and water are converted 

 into organic substances are not fully understood at present. We 

 only know the final products, not the stages that lead up to them. 

 The first substance formed in the plant by the constructive 

 activity of the chloroplasts is some form of sugar. It is most 

 likely cane sugar. If more sugar is produced than can be 

 carried away in the sap, it is converted into starch by the 

 chloroplasts. 



If the green parts of any plants are exposed to light, 

 assimilation commences, and starch appears in the chloroplasts. 

 If assimilation ceases, as it does regularly at night, the starch 

 disappears. The disappearance of the starch is due to a 

 ferment called diastase, which is found in small quantities in 

 various parts of plants. A ferment is a compound which can 

 act on another substance, and convert it into a different material. 

 Ferments may be living or non-living. The Yeast plant is an 

 example of the former and diastase of the latter. 



A very interesting experiment can be performed to show that 

 assimilation has taken place in a green leaf. From a piece of 

 tinfoil, cut out the word " Assimilation," and encase a leaf with 

 it so that the word assimilation is on its upper surface. Allow 

 the leaf to remain on the tree for a few days, and then bleach 

 the leaf, and treat it with iodine solution. The word assimila- 

 tion will appear on the leaf. The whole of the leaf, with the 



