GROWTH BY DAY AND NIGHT. ^^^ 



concerned, only in so far that it must be noticed in the first place that temperatures 

 below the specific zero-point exert no influence on growth whatever, or, put better, 

 do not allow it to take place, and that temperatures beyond the optimum act 

 injuriously. Since, however, in the natural course of things temperatures above 

 the optimum only seldom occur, and in experiments can be avoided, I shall in what 

 follows pass over that entirely, and understand by higher temperatures only such as 

 are below the optimum, and therefore favourable. That even within these limits no 

 simple relation between temperature and the rapidity of growth exists, follows already 

 from Harting's investigations, and J have demonstrated it in detail elsewhere in the case 

 of seedlings; and it is even obvious, because in view of the existence of the grand period 

 and the discontinuous variations of growth, a simple proportionality between growth 

 and temperature is inconceivable, when one and the same part of the plant is con- 

 cerned at different times. In the present state of our knowledge only so much can 

 be said, that, starting from the specific zero up to the optimum, the rapidity of gjrowth 

 is the greater the higher the temperature. 



' When one speaks of the action of temperature on growth, it is tacitly assumed 

 that the temperature indicated by the thermometer also actually exists in the growing 

 part of the plant. Where roots are concerned, which are growing in soil and round a 

 thermometer stuck between them in the soil, the assumption is certainly warranted ; 

 such is not the case, however, when the temperature of the air, according to a 

 thermometer suspended in it, is compared with the growth- of a part of a plant in the 

 air. Since the bulb of the thermometer as well as the part of the plant concerned 

 owe their temperature to the conduction and radiation of heat, and these are certainly 

 materially different in the two cases, for this reason alone it will occur but rarely 

 that the temperature of the growing tissue is exactly given by the thermometer 

 suspended near it To this is to be added that in an atmosphere not completely 

 satxurated with aqueous vapour the plant transpires, and thereby becomes cooled, and 

 this does not lake place in the case of the dry thermometer ; on the other hand, how- 

 ever, it is certain that a wet thermometer will be much more cooled by evaporation 

 from its surface than the plant, the evaporation from which is much smaller in relation 

 to the surface and mass. Unless the opportunity occurs therefore to plunge the thermo- 

 meter, into the observed internode itself (and that has never been done so far, and is 

 impossible in the case of small plants), the thermometer near the plant gives the tempe- 

 rature of the latter only in a very unsatisfactory manner. If the observation is made in 

 the open air, in draughts and rapid variations of temperature, or under circumstances 

 where the plant observed receives the direct rays of the sun, the temperature of the plant 

 will often be very different from that of the thermometer ; but even this source of error 

 is reduced to a minimum when the observation is made in a room with still air, slow 

 and small variations of temperature, and in diffuse light. I shall indicate subsequently 

 the means which I employed to render this error of observation as slight as possible. 



' Quite apart from the fact that under certain circumstances the temperature of a 

 growing sub-a,erial part of a plant may be altered also by the temperature of the water 

 taken up by the roots, and by the exchange of heat with the soil, the influence of the 

 soil is of importance in yet another connection. If the air, and with it the part of 

 the plant above ground, imdergoes rapid and large variations of temperature, these 

 make themselves felt only slowly and to a less extent in the soil and roots ; by this 



