39] 



CHAPTER I. GENERAL PHYSIOLOGY. 



161 



freeze in the interior of the cells. The water which has thus 

 escaped and frozen forms an incrustation (Fig. 122), consisting of a 

 number of elongated ice-crystals arranged side by side. This ice 

 is very pure, for the substances in solution in -the cell-sap remain 

 behind in a more concentrated form. 



The effect on the trunks of trees of exposure to cold is to cause 

 radial splits, which close up again as the temperature rises, the 

 which actually heal only in the cortex. The splitting is due to 

 the unequal contraction of the wood, which is greater in the 

 external more watery portion, than in the interior. 



JLiGHT. The influence of light may be considered under t_wp_ 

 heads : (1) the chemical effects, pro- 

 duced for the most part by the less 

 refrangible rays of the spectrum ; 

 (2) the mechanical effects, produced 

 mainly by the highly refrangible 

 rays. 



The most conspicuous chemical 

 effects are manifested in plants which 

 normally contain chlorophyll. They 

 are : 



a. The formation Of chlorophyll : Pls ' ^--Transverse section of a 



*- frozen leaf-stalk of Cynara Scolymus: 



in Phanerogams the Colouring-matter 6 the detached epidermis ; g the paren- 



in which lie the transverse 

 sections of the vascnlar bundles (left 

 white) ; K K the incrustation of ice 

 Of Conifers and SOine Other plants), consisting of densely-crowded prisms 

 , , - v -,1 , .. ,. -, ' (the cavities of the ruptured tissue 



but remains yellow (etiolm), unless are i e tt black in the figure), 

 exposed to light of not too great in- 

 tensity. This effect is not confined to the rays of low refrangi- 

 bility, but is produced (with equal intensity of light) also by those 

 of high refrangibility. The formation of chlorophyll is also de- 

 pendent on temperature, and will not take place if it be too low ; 

 hence the shoots of plants developed in the early spring remain 

 yellow if the weather is cold. 



b. The assimilation of carbon dioxide by the chloroplastids will 

 only take place in the presence of light of considerable intensity ; 

 it is especially a function of the rays of low refrangibility, as will 

 be subsequently explained. This is also true of the first steps in 

 the assimilation of mineral nitrogenous food (nitrates). 



The most conspicuous mechanical effects, exhibited by plants of 

 all kinds are : 



of the chloroplastids cannot acquire cb5 [ r 

 its green hue (except in the seedlings 



