THE L1FE-HI,ST0KY OF PLANTS. 



369 



the case is reversed — the greater enei-gy then is exer- 

 cised in the subdivision of cells and the consequent 

 growth and formation of new or additional tissue, 

 for the formation of which the food-mateiial stored 

 up during the day is absolutely essential. Tliere is, 

 therefore, in plants a well-marked intermission and 

 variation of growth hy day and by night respec- 

 tively. How the saves become emj)tied of their 

 starch at night has been told in a previous section ; 

 that starch is conveyed to the store organs or to the 

 growing points, and is either stored up or directly 

 applied to the grow- 

 ing tissues. But it 

 may be said that 

 plants grow by day 

 as well as by night ; 

 such growth can be 

 proved by observa- 

 tion and experiment. 

 This is quite true, 

 still the greater 

 amount of growth is 

 hy night. 



Forcing. — 



Growth by day, or 

 at any period, more- 

 over, is easily ac- 

 counted for where 

 there is an available 

 store of food-material 

 to draw upon. Thus 

 in the early stages of 

 forcing operations, 

 provided the heat 

 and moisture be suffi- 

 cient, light is of re- 

 latively little conse- 

 quence. The woi-k 



effected by light was completed last season, and 

 through its agency at that period a sufficient store of 

 food-material was accumulated. But in the case of a 

 weak seedling plant, or of any other in which there 

 is little or no store of food, growth can hardly go on 

 at all during the day. Such energies as the plant 

 possesses must then be devoted to laj^ng in a provi- 

 sion during the day to be utilised at night. Again, 

 growth may be facilitated on a du.ll day in summer 

 by the heat and moisture of the atmosphere, while 

 the amount of light may be inadequate to exert any 

 restraining influence. On the other band, at night, 

 while the absence of light is favourable to growth, 

 the diminished temperature is less advantageous. 

 The comparatively even temperature of the soil 

 must also exert a marked regulating influence. The 

 truth is that with living beings the conditions are 

 48 



Fiar. 52.— Experiment to show the Downward Tendency of the 

 Roots evtn when provided with moisture above, showing that 

 moistnre alone does not influence their direction. 



so mixed in their incidence, and the a(;ti(jiis of 

 plants are so controlled by conflicting circumstances, 

 that it is rai'cly any absolute or dogmatic statement 

 can be made. The spii"it of compromise and recipi-o- 

 cal adjustment reigns supreme in nature, and the 

 more fully the cultivator recognises this and puts it 

 into practice the more successful he is lik(^ly to be. 

 Tho i)hysiologist in the laboratory, for liis purposes, 

 is constantly endeavouring to unravel and do away 

 with these compromises, in order that he may the 

 more clearly ascertain what is the separate effect of 



heat, of light, or 



what not, indepen- 

 dently of other agen- 

 cies; and conse- 

 quently he is in a 

 position to make 

 dogmatic statements 

 based upon what he 

 sees in the labora- 

 tory. But unfortu- 

 nately he is not able 

 to tell us much of 

 what goes on in the 

 living plant under 

 natural circum- 

 stances, like the 

 practical cultivator, 

 he only sees the re- 

 sults, although his 

 previous knowledge 

 gives him a better 

 opportunity for un- 

 derstanding how 

 those results are 

 brought about. 



It may then be 

 certainly assumed 

 that light retards 

 growth, and consequently that growth is most 

 active at night, though it may be modified and con- 

 trolled by other agencies. 



Rate of Growth. — Taking the daily life of a 

 plant, the energy of growth rises from a minimum to 

 its culminating point, and then descends ; and this 

 "curve of growth " is found to be especially favoured 

 by a certain amount of heat, the exact amount vary- 

 ing in the case of different plants, and in the case of 

 different parts of the same plant. The most suitable 

 temperature can only be ascertained by experience, 

 because the meteorological details obtained from 

 a study of the climate of the plant's native country 

 are only suggestive, and afford no direct indication 

 of the power a plant has of adapting itself to the 

 altered circumstances of cultivation. The heat in 



