490 Plant Case for growing Plants 



exists in a state perfectly dry, neither the seed nor the plant can 

 exert that action on the air which is essential for its develope- 

 nient and growth. It is " owing, therefore, to the prevention of 

 the escape of the moisture within the cases, as Mr. Ward ob- 

 serves, that plants will grow in them for many months, or even 

 years, without requiring fresh supplies of water : thus the Poa 

 and Nephrodium, above mentioned, grew for four years in the 

 bottle without receiving one drop of fresh water, and would, as 

 he believes, have grown as many more had they not perished 

 from an accident." In vegetation in the free atmosphere, the 

 fluids, which may be absorbed either from the soil by the roots, 

 or from the atmosphere by the leaves, are, in great part, ex- 

 haled and dissipated ; but, in the plant cases, they are condensed 

 on the inner surface of the glass roof, and fall back to the soil 

 from which they were raised. In this way, both the soil and 

 atmosphere possess always sufficient humidity to carry on vege- 

 tation. 



The degree of humidity which is thus maintained is not, 

 however, suited to all plants. Those which partake largely of 

 a cellular structure, and possess a succulent character, and 

 especially those which have fleshy leaves, bear best the atmo- 

 sphere generally existing in these cases ; whilst, on the contrary, 

 its continued humidity is unfavourable, says Mr. Ward, to the 

 developement of the flowers of most exogenous plants, except 

 such as naturally grow in moist and shady situations. If, indeed, 

 we call to mind the vast quantity of moisture which many plants 

 naturally exhale in the free atmosphere, and the check which 

 their vegetation receives if the atmosphere continue for some 

 time both humid and still, we cannot wonder that to such plants 

 the moist air of these cases should be unsuited, and that many 

 of them, placed in such circumstances, should, as it is said, 

 " damp off." But others of a different character, whether 

 growing in the soil, or suspended from the roof, find always 

 sufficient moisture to support a healthy vegetation. Hence the 

 supply of water given to the soil in the first instance, being 

 secured from waste, is successively absorbed, exhaled, and con- 

 densed within the case itself, and made to sustain over and over 

 again the vegetation of the same plants, without suffering either 

 the soil or the atmosphere to become, at any time, too dry to 

 carry on that process. 



6. Condition of Plants, in regard to Heat, in close Cases and in 

 the free Atmosphere. 



The condition next to be noticed is that which relates to 

 temperature. In the list of plants growing together in these 

 cases are some which are natives of the tropics, others which 

 have been brought from high latitudes, and others the growth 



