BOTANICAL TKUMS. 



47 



A-ssimilation. 



Tliis is the process by which the carbon obtaiiiod from carlion 

 dioxide is combined with the elements of wiiler to form 

 starch. 



Metastasis. 



This is the process by wliich the starch, resultiiiK from 

 assimilation, is converted into soluble forms and removed 

 from the cells where it was produced to other portions of 

 the plant where it is needed for purjioses of growth, or, if 

 thei-e is an excess, to storehouses sucli as roots, bulbs, 

 etc., for future use. 



Circumstances Afifecting Growth. 



Temperature. — Growth maj' bo stoitiied altof^ether by either 

 too low or too high a temperatwro, and between the lim- ] 

 its within^vhich any given plant is found to be capable 

 of growth there will be found a particular degree of 

 temperature more favourable to growth than any other, 

 either above it or below it. This may be called the apti- 

 mum. The effect of temperature differs considerably ac- 

 cording to the amount of water present in the part 

 affected, dry seeds, for instance, resisting a temperature, 

 either high or low, to which soaked sei».ds would at once 

 succumb. 



Liylit. — Light is essential to assimilation, but seeds and 

 tubers, as well as nianj- of the lower plants which are 

 without chlorophyll, such as Mushrooms, will grow in 

 the absence of light as long as the stock of assimilated 

 material upon which they draw is not exhausted. Tlie 

 growth which takes place in the cambium-layer of dico- 

 tyledons and in roots is another example of increase in 

 size in the absence of light. The assimilated material in 

 all these cases, however, has been previously elaboratwl ; 

 elsewhere. j 



Light is found to exercise a retarding influence upon 1 

 growth. A plant, for instance, in a window will bend j 

 towards the light, because the cells on the side nearest | 

 the window grow more slowly than those which are 

 shaded, thus causing curvature of the stem and petioles. 



Gravitation. — Grva.\\ta.t\on also affects growth, as we know , 

 that the stem and root, or axis of the plant, are usually | 

 in the line of the radius of the earth at the place of j 

 growth. If a seedling plantlet be laid with the stem and | 

 root horizontal, the stem will curve upward and the root 

 downward in the endeavour to restore the vertical direc- | 

 tion. 



