DISEASES. 1C7 



rendered abortive by the production of male flowers only, 

 when exposed to too great heat, and by an opposite treat- 

 ment, when subjected to a low temperature for a long 

 time, others produced only female flowers. In some 

 plants a high degree of heat, with moisture, results in the 

 production of leaves only, and Humboldt found that 

 wheat was grown about Xa,lapa, Mexico, as a fodder 

 plant, because it produced an abundance of grass, but did 

 not form ears nor grain. 



A diminished temperature, on the contrary, removes 

 the stimulus of growth, and leads to the suspension of all 

 vital action in proportion to its reduction. At the freez- 

 ing point it is probable that all such action ceases, though 

 in this regard there is great differerica among plants ; the 

 mosses and lichens w 11 flourish, and the Chickweed will 

 vegetate and blossom at a temperature very little above 

 freezing. The access of frost, after vegetation has some- 

 what advanced, often proves very disastrous, and we not 

 unfrequently lose our crops of fruit by such an oociinsence 

 during the period of blossoming, or even afterward. 



Some plants in a dormant condition, will endure unin- 

 jured a great depression of temperature, while others will 

 be destroyed by the slightest approach of frost. Accord- 

 ing to De Candolle, this may depend upon the greater or 

 less amount of water they contain, upon the greater or 

 less viscidity of their fluids, or the rapidity with which 

 these fluids circulate. Those with larger cells he thinks 

 most easily injured by frost, and those which contain a 

 great deal of air are able to resist it best. The freezing 

 point will vary according to the quality of the sap, for we 

 know that difllsrent vegetable juices congeal at different 



