VARIETIES IN DEGREE OF EXHALATION. 175 



mistaken for dew. This, however, is not the case ; since it has 

 been observed on plants under shelter, as well as on those that 

 are exposed ; and it has been noticed also at other parts of the 

 day. A similar accumulation of water in drops has been ob- 

 served when plants have been electrified ; by which process the 

 amount of exhalation appears, for a time at least, to be consider- 

 ably increased. It is perhaps in this manner, that an electric 

 state of the atmosphere hastens the growth of some kinds of 

 plants. 



266. If plants are exposed to a light of too great intensity, 

 especially if they possess many stomata, and are not well supplied 

 with water, their tissue becomes dried up by the increased ex- 

 halation which then takes place, and which is not sufficiently 

 counterbalanced by absorption, so that their vegetation is mate- 

 rially checked, a fact of which we see abundant examples in 

 dry sandy soils, and exposed situations. If, on the other hand, 

 the leaves are shaded, and the roots freely supplied with mois- 

 ture, the growth of the plant is active and luxuriant, but its 

 tissue is soft, and altogether destitute of firmness. This, how- 

 ever, is partly due to the imperfect performance of another 

 process shortly to be described as that of digestion. Plants of a 

 very fleshy juicy character, termed succulent, in which there is 

 usually a great deficiency, or even entire absence, of stomata, 

 require a considerable amount of light to secure for them that 

 regular discharge of moisture which they require ; hence when 

 Melons are grown in a frame, as many leaves as possible should 

 be exposed to the influence of the sun's rays, and the accumula- 

 tion of moisture within should be provided against. There are 

 certain succulent plants, which, owing to their deficiency of 

 stomata, may be preserved without moisture for many days or 

 even weeks ; and as their cuticle is so thick as to resist evapora- 

 tion, it is often very difficult to kill and dry them for the pur- 

 pose of placing them in collections. Of this kind are the Sedums 

 or Stone-crops, of Britain, which have been known to push con- 

 siderable shoots when placed under pressure; and many plants 

 of tropical climates. 



267. Besides these applications of theory to practice, there 



