VEGETABLE LIFE. 17 



aqueous sap are more liable to be killed by frost 

 than those charged with a gummy or resinous juice. 

 So those which are growing freely suffer more than 

 such as are dormant. Some plants, as young wheat 

 for instance, are only withered by frost, without 

 laceration of their cellular structure ; as on the 

 return of warmth they regain their rigidity. 



Extreme drought is fatal to the vitality of all 

 plants ; exhaustion of the juices by evaporation, 

 causes an injurious shrinking of the vascular apparatus, 

 and consequent morbosity of the organisation. Some 

 plants are naturally defended against drought by the 

 thick pulpy substance of their stems and leaves, such 

 are what are called succulents : others are protected 

 from the heat of summer by protuberant appendages 

 of their stems ; these are bulbs and tubers : and 

 there are certain plants, which, if accidentally placed 

 on a soil too dry for their constitution, form for 

 themselves cellular protuberances at the base of their 

 stems, to serve as provisional reservoirs of moisture ; 

 such is the Phleum prafense. 



Although heat and moisture be absolutely neces- 

 sary to vegetable development, they are of them- 

 selves inadequate to cause even the germination of 

 seeds ; because such as lie deeply buried in the earth 

 receive no stimulus from the heat and moisture with 

 which they are there surrounded : air and its quali- 

 ties are wanting; and without which no develop- 

 ment can take place. If seed be exposed to perfectly 

 dry air, it receives no excitement therefrom ; so that 

 c 



