303 '' LECTURE XVIII. 



assimilation by means of the decomposition of carbon dioxide, in most plants, and 

 especially in the case of meadow- and cultivated plants, trees, and garden-plants of 

 the most various kinds, only takes place with normal vigour and productiveness 

 when the ordinary strong daylight of summer is at the disposal of the plants. The 

 much feebler light in greenhouses, or even in ordinary dwelling-rooms, suffices, it is 

 true, with most plants to bring about a less productive assimilation in the green 

 leaves; but the sickliness of the plant shows how feeble is the nutrition under 

 such circumstances. It is also to be observed that a pot plant standing close to 

 a window, under the best of circumstances only receives the Jight radiating from 

 half the sky, and only meets with the direct rays of the sun occasionally. If the 

 plant stands somewhat further removed from the window, it is only necessary to 

 imagine straight lines, running from the plant or a leaf to the edges of the window, 

 and thence direct to the sky, to find the extent of that part of the latter the rays 

 of which fall directly on the leaves : it is then perceived that a plant removed but 

 a few metres from the window, only receives a very small proportion of light from 

 the sky, and as a rule meets with no direct sunlight at all. Accordingly, the 

 nutrition of plants in the middle of a room is extremely poor, and sooner or 

 later they inevitably perish. On the other hand, however, it is also to be observed 

 that while there are many plants which only flourish well in places which receive 

 the full light from the sky, and the direct rays of the sun, others exist which 

 prefer the shade of woods, or even the feeble illumination in the interior of deep 

 caverns. Here belong, for example, besides some species of Pyrola, many 

 Mosses, and especially Liverworts ; those Algae which grow exclusively In the 

 depths of large seas, and are thus feebly illuminated, also show that they find 

 the conditions of their existence in less intense light. Just as for each mani- 

 festation of life in plants there is an upper limit of temperature, which cannot 

 be passed over without injury, so also there is certainly an upper limit of intensity 

 of the light, at which the chlorophyll-grain can no longer accomplish assimi- 

 lation. Of course this limit of the intensity of light cannot be exactly given, 

 in the absence of suitable photometric methods; and when Pringsheim makes 

 circumstantial statements concerning the behaviour of cells containing chlorophyll 

 in the focus of a lens, or in the sun's image, as he terms it, these purely 

 pathological processes have about as much physiological value as if, for any 

 reason whatever, a so-called sun's image were allowed to act on the retina of the 

 eye through a burning-glass. Much better are the statements of several ob- 

 servers who, employing direct light, allowed the evolution of oxygen of one and 

 the same plant to take place under various degrees of shading, and so established 

 that a maximum effect at a light-optimum exists for this function also. In the 

 absence of photometric measurements of general value however, I pass over these 

 statements also. 



We have much more information as to the various effects of the individual con- 

 stituents of sunlight, than with respect to the cardinal points of the intensity of the light 

 concerned in assimilation. As is well known, the light of the sun, Uke that of most 

 incandescent bodies, is a mixture of very different luminous rays, which are distinguished . 

 by their refrangibihty, i.e. by the amount of divergence which they undergo on entering 

 another medium, as well as by their chemical effects ; and obviously the question must 



