THE GARDENER. 



9 



fects in the enlargement of the bulb; but in a drier 

 climate, such as that of France, the same turnip does 

 not succeed. It depends much on the action of its 

 leaves, which in very dry air exhale more moisture 

 than a limited supply of moisture can replace ; they 

 then become constricted, and consequently the growth 

 and succulence of the whole plant become injuriously 

 affected. 



Experience teaches us that heat combined with 

 moisture causes the greatest vegetation ; as for exam- 

 ple, at Guiana, where the sun is always extremely 

 powerful, and the air humid. Though a moist atmo- 

 sphere of from 48° to 50° is sufficient for most plants, 

 the great family of them have a nature and habits 

 adapted to a temperature from 75° to 85° ; or even 

 100° by sun heat, \yhen we see vegetation failing 

 from excessive heat, it is because the moisture of the 

 earth, or the air, or of both, is insufficient. It is on 

 our knowledge of this that the theory of watering 

 depends; and in an analogous way we learn by our 

 familiarity with other general principles of vegetable 

 physiology, how to exercise practical modes of treat- 

 ment with the utmost probabilities of success ; whereas 

 otherwise we should be in the dark as to certain ope- 

 rations of practice until experience had w^arranted 

 their adoption, and in many cases valuable plants 

 would be lost before this experience could be possessed. 



Professor Lindley, in his delightful work on the 

 Theory of Horticulture, remarks, that by examining 

 the structure of a leaf through a microscope, the na- 

 tural habits of unknown plants may be ascertained 

 with sufficient accuracy. If they be provided with 

 thick epidermis (or outer covering), with only a few 

 pores, they ought to inhabit situations where the air 

 is dry, and but little liquid nourishment can be ob- 

 tained. On the other hand, if their leaves are pierced 

 with numerous and large stomates (pores), these 



