416 Prof. H. Marshall Ward. The Relations between 



plant. I need not remind you that some rays of the solar light, 

 especially some of the less refrangible (orange and red) rays, are more 

 concerned in the process of assimilation than others, and, although we 

 uannot here stop to discuss this matter in detail, I may point out that, 

 as different rays of light are absorbed or reflected in the atmosphere, 

 we may have variations in this connexion of more or less importance 

 to the plant. The experience of photographers shows that the different 

 thicknesses of the atmosphere through which the light has to pass, 

 reflection from a cloud as contrasted with the " blue sky," &c, all 

 exert influence on the composition of the light, and in prolonged 

 'cloudy, dull weather or fogs this factor may add its effects to those 

 ' due to the mere dilution of the light as a whole. 



But the fundamental nature of the necessity for a suitable intensity 

 of light of the right composition is best brought out in studying the 

 effects of low intensities of light on the green organs of plants. 

 *> As is well known, the general effect of keeping a plant in the dark 

 is to induce a condition known as etiolation.* The whole plant 

 becomes pale yellow or colourless, and has a curiously trans- 

 lucent, watery appearance ; the internodes are excessively long, while 

 the leaves, on the contrary, are usually small and crumpled. Closer 

 investigation shows that each cell of the internodes is abnormally 

 elongated, its cell- wall thinner than usual, and its chlorophyll corpus- 

 cles small and wanting the green chlorophyll. If we examine the 

 vascular bundles, they are found to be deficient in firmness, because the 

 substances which normally go to thicken their walls have not been 

 forthcoming. 



Everything about the etiolated shoot indicates tenderness, and as 

 a matter of fact snch shoots are very ill-adapted to withstand the 

 ordinary exigencies of plant life. Undoubtedly the chief cause for this 

 weak condition is the absence of the light necessary for the purposes 

 of assimilation ; the carbon dioxide maybe present, and even the fully 

 green chlorophyll could be developed by a few hours' exposure to feeble 

 light, but these do not suffice for the construction of the materials 

 such as glucose, starch, &c, necessary to enable the protoplasm to 

 keep the tissues normal. 



Nevertheless, the other functions of the cell are being carried on 

 with remorseless pertinacity. The oxygen of the air enters the pro- 

 toplasm, establishes its usual combinations, and carbon dioxide and 

 water are given off to the air. The chemical changes known collec- 

 tively as metabolism proceed, and result in the addition of substances 

 to the cell- sap which were not previously there. To an extent more 

 marked than ever before, the turgid cells may be elongating, and this 



* See Sachs (' Bot. Zeitg.,' 1863, supplement, and 1865, col. 117, &c.) ; Kraus 

 (' Jahrb. f. wiss. Bot.,' vol. 7, p. 209 ; G-odlewski ('Bot. Zeitg.,' 1879, col. 81) ; de 

 Vries ('Bot. Zeitg.,' 1879, col. 852) ; Oodlewsk ( Biol. Centralbl.,' 1889). 



