CHAPTER I. — GENERAL PHYSIOLOGr. 673 



thus escaped and frozen forms an incrustation (Fig. 467), con- 

 sisting of a number of elongated ice-crjstals 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. 



It has been ascertained that this formation of ice is, in itself, 

 not necessarily fatal in all cases. If the frozen part be slowly 

 thawed, the cells may gradually reabsorb the water, and so return 

 to their previous normal condition. If, however, the frozen part 

 be rapidly thawed, the cells cannot absorb the water sufficiently 

 rapidly ; it therefore either collects in the intercellular spaces, 

 causing discolouration and decay, or it runs off and evaporates, so 

 that the part dries up. In many cases, however, slow thawing 

 does not ensure immunity ; for instance, the leaves of the Pump- 

 kin or the Dahlia : it appears that, in such cases, the escape of 

 water from the cell, which takes place at the time of freezing, 

 causes fatal disorganisation of the cells. Again, in the case of 

 long-continued cold, the ice formed outside the cells may be 

 gradually removed by evaporation, so that on thawing, the water 

 necessary to restore the normal condition of the cells is not forth- 

 coming. 



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

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

 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 intei'ior. 



Light. The influence of light may be considered under two 

 heads : (1) the chemical effects, produced for the most part by the 

 less refrangible rays of the spectrum ; (2) the mechanical effectSf 

 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 : in Phanerogams the colouring- 

 matter of the chloroplastids cannot acquire its green hue (except in 

 the seedlings of Conifers and some other plants), but remains 

 yellow (etiolin), unless exposed to light of not too great intensity. 

 This effect is not confined to the rays of low refrangibility, but 

 is produced (with equal intensity of light) also by those of high 

 refrangibility. The formation of chlorophyll is also dependent 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. 



