THE FIRST VISIBLE PRODUCT OF ASSIMILATION. 307 



erroneous assuniption that this blue light must be useful to the plant because it 

 belongs to the so-called chemical part of the spectrum. It has been shown above 

 that the latter is not the case ; and moreover blue glass at most transmits as much 

 blue light as colourless. That it appears blue is simply because it destroys a large 

 proportion of the yellow light; and we have learnt that it is just this yellow light 

 which is the effective constituent of daylight in the nutrition of the plant. The 

 effect of glazing greenhouses with blue glass is therefore directly injurious : it 

 depends on false conclusions from inaccurately established facts. 



Finally, the question as to the action of different coloured light on the nutrition 

 of plants comes into consideration when artificial terrestrial sources of light are 

 employed for the illumination of plants instead of ordinary sunlight. This has 

 hitherto taken place, of course, only in the interest of scientific research. It is 

 theoretically certain, and has been established by experiment that the electric light 

 — i. e. the light from incandescent electrodes — as well as lamplight, &c., when 

 sufficiently intense, can effect the evolution of oxygen from organs containing 

 chlorophyll. Here, however, it is not to be forgotten that every such source of 

 light possesses a different mixture of rays of different refrangibility, or, in other words, 

 a different spectrum ; and, from what has been said previously, each source of light 

 particularly rich in yellow rays will cause more vigorous assimilation. 



We pass, finally, to the fourth question mentioned in the introduction, which 

 inay be shortly answered thus : the first definitely established product of assimilation 

 in the chlorophyll-corpuscles is starch (Amylum), or some other substance equivalent 

 to it. Very indefinite ideas were formerly entertained in this connection. It was 

 supposed, as resulted from the statements of the older physiologists and chemists, 

 that an indefinite primordial slime, perhaps composed of various substances and 

 impossible to characterise in detail, arises in the green organs of plants : it was 

 believed that this substance was again recognised on its way in the living cortex 

 of trees, and it was assumed that it penetrated into the various organs and tissues, 

 there to break up into the numerous and various chemical compounds which 

 are found in different parts of the plant. In a long series of micro-chemical and 

 experimental researches on the distribution of starch and sugar, of proteid substances 

 and of fats, and their use in growth, I came to the conclusion in 1862' that 

 the enclosed starch, which had already been observed in the chlorophyll-corpuscles 

 by Naegeli and Mohl, is to be regarded as the first, evident product of assimilation 

 formed by the decomposition of carbon dioxide. I said to myself, if this view 

 is right, the formation of starch in the chlorophyll-corpuscles must cease on 

 the exclusion of light, since the decomposition of carbon dioxide can then no 

 longer take place ; and that in like manner renewed access of light to the chloro- 

 phyll-corpuscles must also bring about a renewal of the formation of starch in them. 

 These and similar deductions were confirmed by appropriate investigations; and 

 yet more — I had already concluded from my previous micro-chemical analyses 



' The foundations of my theory, that the starch in the chlorophyll is the first evident 

 product of assimilation, were first laid in the following of my works: — ^ Flora! 1862, Nos. 11 and 

 21 ; and 1863, p. 33 ; ' Bot. Zeit.^ 1862, p. 366 ; 'Jahrb. fiir wiss. Bot! 1863, III, p. 199 ; ' Exp.- 

 Pkys.' 1865, p. 320. 



