GIVEN OFF BY PLANTS DURING THEIR GROWTH. 



49 



materially due to the external influence referred to, irrespectively 

 of a corresponding enlarged surface and rapidity of fixation of 

 constituents. So far, of course, as the process is one of simple 

 evaporation, will an increase of temperature determine a greater 

 loss of water ; still the question arises whether — supposing there 

 be no actual deficiency of the necessary and available consti- 

 tuents — this increased passage of water through the plants, 

 carrying with it in its course many important materials of growth 

 from the soil, and probably also influencing the changes in the 

 leaves of these, as well as of those derived from the atmosphere, 

 will not be accompanied with an equivalently increased growth 

 and development of the substance of the plant. Upon this point 

 some light may be thrown by an examination of the circum- 

 stances attending the development of u Roots," the more active 

 growth of which is generally coincident with a declining and not 

 an increasing temperature, as in the case of the seeding crops 

 now under trial. 



Until, however .ne relationship of the quantity of water given 

 off to the amount of dry substance or its constituents fixed, under 

 varied and known circumstances, be experimentally determined, 

 any detailed consideration of the indications of the thermometer 

 in connection with results of so initiative a kind would be 

 unavailing, though a more or less complete registry was kept of 

 the temperature during the period of the growth of the plants, 

 and this point will not be neglected in our future progress. 



As might have been anticipated, it is seen by the tables, that 

 though, as the season advanced in temperature and the mass 

 and surface of the plants increased, the amount of water daily 

 given off was also greater up to a given time, yet towards the 

 end of the experiment it rapidly and considerably diminished. 

 It is probable that from the time of this apparent decline in the 

 rate of passage of water through the plant the processes of acquire- 

 ment of material were less active, those of the ripening and 

 elaboration of its contents having commenced, and that the time 

 of most active circulation as indicated by the daily rate of 

 water evaporated was also that of the greatest accumulation. 

 Some experiments which we conducted a few seasons ago, with 

 the view of determining whether there was, in the formation and 

 ripening of the cereal grains, any diminution in the amount of 

 nitrogen previously stored in the plant, seemed, indeed, to show, 

 that though there was no appreciable change in the amount of 

 the nitrogenous compounds upon a given acre of land after the 

 time of flowering, yet the amount of mw-nitrogenous vegetable 

 substance accumulated after this time had been very great ; it is 

 not, however, necessary to conclude that the rapid accumulation 

 of carbon from the atmosphere at this period, though coincident 



VOL. V. E 



