46 Elementary Biology. 



be borne in mind in this relation that many chemical changes 

 are furthered by the rays of high refrangibility. For example, 

 the decomposition of silver salts in photography takes place 

 under the influence of the violet rays of the spectrum. 

 Probably special rays a 1 through the spectrum are capable 

 of bringing about or influencing in some way or another 

 special chemical changes, while only some (those beyond 

 the red end of the spectrum) are capable of producing the 

 sensation of heat, and others (the red and yellow) the 

 sensation of light, in any appreciable degree. 



In reference to the decomposition of carbonic acid by 

 chlorophyll-bearing cells, it is found that the less refrangible 

 rays are those through whose influence carbonic acid is 

 decomposed and chlorophyll is produced, while the more 

 refrangible rays have not that effect. 



The influence of light on plants varies with its intensity. 

 This subject is, however, still far from being thoroughly 

 investigated, so that little may be said on this subject beyond 

 the general statement that diffused daylight seems more 

 suitable for the furtherance of plant life than direct sunlight, 

 while assimilation ceases after sunset. 



Manifestly this subject is closely connected with the depth 

 to which the light penetrates into the plant tissues, since 

 the light loses in intensity according to the thickness of the 

 tissue through which it has passed. The rays of least re- 

 frangibility, that is to say those chiefly concerned in chemical 

 changes in plants, are found to penetrate most deeply. The 

 rays at the violet end of the spectrum are in great part 

 absorbed by the chlorophyll and the colouring matters in the 

 superficial cells. In regard to aquatic, and especially marine, 

 plmts, the depth to which light penetrates through water has 

 to be considered. Experiments have shown that darkness 

 prevails at all depths beneath 100 fathoms, and conse- 

 quently that no vegetal life can exist beneath that depth, if, 

 indeed, it ever exists so far. 



(b) In regard to animal life, the presence or absence of 



