209 



they usually grow. Again, as regards the stage of de- 

 velopment, we know that many trees which are easily 

 killed by extremes^ of temperature as seedlings are 

 resistant when older, and that, for example, flowers and young 

 leaves may be readily injured while resting buds are very 

 resistant. The quantity of moisture also present" in a plant 

 materially affects its power of withstanding extremes of tem- 

 perature. Many mosses and lichens are easily killed when 

 moist, but are extremely resistant when dry. The more water 

 contained in plant tissues the more likely are they to be 

 injured by frost. 



188. Excessive heat most com- Effects of 

 monly injures plants in one of the following ways : Excessive 



.ti.Cii 



(1) By causing the drying up and death of leaves, twigs, 



branches, or even entire plants and trees, owing to 

 the abstraction of water from the plants themselves 

 and from the soil in which they grow. The roots 

 are thus unable to obtain sufficient moisture from 

 the soil to make good the losses occasioned by 

 increased transpiration. This form of injury is 

 most destructive in India in years when the more 

 or less complete failure of the monsoon has lowered 

 the level of all springs and considerably reduced 

 the amount of moisture ordinarily available in the 

 soil. 



(2) By splitting the stems of trees. Intense heat, drying 



up the outer tissues of a stem and thus causing them 

 to shrink, gives rise to splits in the wood very 

 similar to frost cracks. Thin-barked species, such 

 as Phyllanthus Emblica, may thus bo found split 

 to the centre, especially in years of drought. 



(3) By directly scorching and killing the cambium and 



other living tissues, thus causing cancerous patches 

 on the stems, or the death of entire trees. This 

 is often caused by the sun's rays impinging on 

 thin-barked stems, especially those which have 

 grown up in shade and then become exposed by 

 the felling of the surrounding growth, as happens in 

 the case of standards in a coppice. 



189. Excessive cold usually results Effects of 

 in one of the following modes of injury : 



(1) The direct freezing and killing of parts of plants, such 

 as the leaves, twigs, etc., and even of entire plants 



