100 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 



of their small number of stomata ; whilst, on the other hand, they absorb 

 with great readiness during rainy weather, and are enabled, by the 

 fleshiness of their substance, to store up a large quantity of moisture 

 until it is required. In some parts of Mexico, the heat is so intense, 

 and the soil and atmosphere so dry, during a large part of the year, 

 that no vegetation is found at certain seasons, save a species of Cactus ; 

 this aifords a wholesome and refreshing article of food, on which travel- 

 lers have been able to subsist for many days together, and without 

 which these tracts would form impassable barriers. On the other hand, 

 the plants of damp situations usually exhale moisture almost as fast as 

 they imbibe it ; and consequently, if their usual supply be cut off or 

 diminished, they soon wither and die. Plants that usually live entirely 

 submerged, are destitute of the cuticle or thin skin, which covers the 

 surface in other cases ; in consequence of this, they very rapidly lose 

 their fluid, when they are removed from the water ; and they are hence 

 dependent upon constant immersion in it for the continuance of their 

 lives, although their tissues may not be remarkable for the amount of 

 fluid which they contain. 



154. There are some Plants which are capable of adapting themselves 

 to a great variety of situations, differing widely as to the amount of 

 moisture which their inhabitants can derive from the soil and atmo- 

 sphere ; and we may generally notice a marked difference in the mode 

 of growth, when we compare individuals that have grown under oppo- 

 site circumstances. Thus a plant from a dry exposed situation, shall 

 be stunted and hairy, whilst another, of the same species, but developed 

 in a damp sheltered situation, shall be rank and glabrous (smooth). 

 But in general there is a certain quantity of moisture congenial to each 

 species; and the excess or deficiency of this condition has, in conse- 

 quence, as great an influence in determining the geographical distribu- 

 tion of Plants, as the amount of light and heat. Thus, as already 

 remarked, the Orchidese and Tree Ferns of the tropics grow best in an 

 atmosphere loaded with dampness ; whilst the Cactus tribe, for the most 

 part, flourishes best in dry situations. The former become stunted and 

 inactive, if limited in their supply of aerial moisture ; whilst the latter, 

 if too copiously nourished, become dropsical and liable to rot. Among 

 the plants of our own country, we find a similar limitation ; a moist 

 boggy situation being indispensable to the growth of some, whilst a dry 

 exposed elevation is equally essential to the healthy development of 

 others. There is a beautiful species of exotic Fern, the Trichomanes 

 speciosum; the rearing of which has been frequently attempted in this 

 country and elsewhere, without success ; but which only requires an 

 atmosphere saturated with dampness, for its healthy development, being 

 easily reared in one of Mr. Ward's closed glass-cases. In this, as in 

 similar examples, it is only necessary to imitate as closely as possible 

 the conditions under which the species naturally grows ; and sometimes 

 this can only be accomplished, by surrounding the plant with small 

 trees and shrubs, so as to give it a moister atmosphere than it could 

 otherwise attain. Professor Royle mentions the growth, under such 

 circumstances, of a fine specimen of the Xanthochymus dulcis, one of 

 the G-uttiferce or Gamboge-trees, in the garden of the King of Delhi ; 



