414 Prof. H. Marshall Ward. The Relations between 



necessary water. If we accept, with de Vries,* that these substances 

 are chiefly the organic acids and their salts, then we may expect the 

 study of influence of light in promoting the decomposition of organic 

 acids in the plant to give more information on these matters. The 

 same remarks apply with regard to the influence of variations in tem- 

 perature. 



Growth is, of course, impossible without water, and the transpira- 

 tion current supplies this to the osmotically active cells. In nature, 

 the quantity of water at the disposal of these cells varies enormously, 

 not only with the quantities at the disposal of the root-hairs, but also 

 with the rapidity of the transpiration influenced by the atmosphere. 

 On the whole, given favourable temperature, and other circumstances, 

 growth in length is most active in damp weather, when the quantities 

 of water in the cells are relatively very large ; it is retarded in hot, 

 dry weather, because the loss of water is sufficiently extensive to 

 diminish turgidity. 



Passing now to carbon assimilation, I come to the subject which 

 offers most interest for our enquiry. Assimilation is also to some 

 extent influenced by temperature, although in a very different manner 

 from respiration ; f and the influence of even large variations 

 in temperature may be masked by the effects of small variations 

 in other factors, especially light. Assimilation takes place at low 

 temperatures whenever respiration is possible, but the temperature 

 curve for assimilation in ordinary bright sunlight is steeper 

 than that for respiration, and at higher temperatures (say, 30° C. 

 and above) where respiration is not yet most active, assimilation 

 is already beginning to decline. In the blackberry, for instance, 

 whereas assimilation is most active at between 29° and 33° C, re- 

 spiration goes on becoming more and more energetic to 46° C, at and 

 beyond which its effects are of course dangerous to the plant. % On 

 the whole, we may conclude that at low temperatures, say, 5° to 10° C , 

 on a bright spring morning, assimilation is relatively more active 

 than respiration, whereas at higher ones — 30° to 40° — the reverse is 

 distinctly the case. 



The effects of variations in the intensity and kind of light on assi- 

 milation have been much studied, and may be summed up generally 

 for our purposes as follows. 



With ordinary solar light, as it reaches the plant on a clear day in 

 the open, the activity of assimilation increases nearly in proportion to 

 the intensity § of the light ; this is usually expressed by saying, the 



* ' Unters. uber d. median. Ursaehen d. Zellstreckung,' 1877, and 'Bot. Zeitg.,' 

 1879, col. 848. 



f See Kreussler, ' Landwirthschaftl. Jahrb.,' vol. 16, 1887, and vol. 17, 1888. 

 X See Kreussler, loc. cit., 1887, p. 746. 



§ The word must not be pushed too far as to meaning, in the absence of any satis- 



