Pretoplasm. 45 



D. Light. (a) In relation to vegetal life, it has already 

 been explained that the life of plants containing chlorophyll 

 is entirely dependent on the action of light, inasmuch as 

 the assimilation of carbonic acid is impossible without it. 

 The higher metabolic processes can, however, go on equally 

 well in darkness. It follows that while these processes are 

 active in all parts of the plant, the assimilation of carbon 

 takes place only in the parts which contain chlorophyll. 

 Plants not containing chlorophyll, such as the parasitic 

 fungi, are dependent on chlorophyll-bearing plants for the 

 products of assimilation on which they subsist. 



Sunlight, as is well known, is a mixture of a number of 

 coloured rays. Sunlight may be readily resolved into its 

 constituent coloured rays by passing it through a prism. 

 In accordance with certain laws of optics, the light comes 

 out of the prism at a different angle to its surface from that 

 at which it enters. Moreover, each of the constituent 

 coloured rays issues at an angle peculiar to itseJf. The 

 rays are said, therefore, to be differently refrangible. The 

 result is, consequently, that the broken-up ray of white 

 light forms a band of colour known as the solar spectrum. 

 The colours are red, orange, yellow, green, blue, indigo, 

 and violet, and are arranged in that order. These colours 

 merge gradually into one another; hence the term 'con- 

 tinuous ' as applied to such a spectrum. The red, orange, 

 yellow, and green are the rays of low refrangibility, while 

 the refrangibility gradually increases through blue and indigo 

 to the violet-end of the spectrum. There is abundant 

 evidence to show that there are other rays, non -luminous, 

 and therefore invisible, beyond the red rays on the one 

 hand, and beyond the violet on the other. 



It is found that these different rays have not all the 

 same effect on vegetal life. For instance, the rays of low 

 refrangibility are those concerned in the chemical changes 

 in the plant, whilst mechanical changes are furthered by the 

 rays at and beyond the violet end of the spectrum. It must 



