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PER 



PL A 



ration are placed on their upper 

 surface ? But that the stems of 

 plants contain some bibulous pores, 

 seems evident from this, that when 

 placed in the earth, they will send 

 out roots. But these pores in 

 the stems are so few, that the stop- 

 ping of them does not materially 

 injure a plant. 



As animals have other ways of 

 throwing ofif those parts of their 

 food which are not fit to nourish 

 them, it is no wonder that plants 

 have been found to perspire insen- 

 sibly a far greater quantity than 

 animals. Plants cannot choose 

 their food as animals do, but must 

 take in that which is presented by 

 the earth and atmosphere, which 

 food in general is more watery, 

 and less nourishing, than that of 

 animals ; and for these reasons, 

 also, it might be justly expected, 

 that the matter perspired by a 

 plant should be vastly more than 

 that perspired by an animal of the 

 same bulk ; and this has been 

 found to be the case. See the ar- 

 ticle Leaves. 



A practical inference or two 

 from the copious perspiration of 

 plants may be, that the plants we 

 cultivate should not be set too 

 close, that they may not be incom- 

 moded, or rendered sickly, by the 

 unwholesome steams of each other. 

 They are as liable to be injured 

 this way, for ought that appears to 

 the contrary, as animals are. And 

 the water that drips from trees 

 upon smaller vegetables is known 

 to be not healthy for them ; the 

 reason is, because this water con- 

 tains some of the matter which is 

 perspired from the trees. But if 



the perspirable matter of plants be 

 injurious to plants, it does not fol- 

 low that it is so to animals. It is 

 thought to be not so in general, 

 but the reverse. The effluvium 

 of poisonous plants is an exception. 



PLANTS. Sir Humphrey Davy 

 says, that every plant examined as 

 to external structure, displays at 

 least four systems of organs, or 

 some analogous parts. First, the 

 Root ,' secondly, the Trunk and 

 Branches, or stem ; thirdly, the 

 Leaves; and fourthly, the Flowers 

 or Seeds. The roots of plants in 

 their anatomical division are very 

 similar to the trunk and branches. 

 The root may indeed be said to be 

 a continuation of the trunk termi- 

 nating in minute ramifications and 

 plamenta and not of leaves. 



The bark is covered by a thin 

 cuticle or epidermis, composed of 

 a number of laminae, or scales. 

 Immediately beneath the epider- 

 mis, is the parenchyma, a soft 

 substance, consisting of cells filled 

 with fluid, having almost always a 

 greenish tint. The innermost part 

 of the bark is constituted of corti- 

 cal layers, and their numbers vary 

 with the age of^the tree. 



It has been shewn by the ex- 

 periments of Mr. Knight, and 

 those made by other physiologists, 

 that the sap descending through 

 the bark, after being modified in 

 the leaves, is the principal cause 

 of the growth of the tree ; thus, if 

 the bark is wounded, the principal 

 formation of new bark is on the 

 upper edge of the wound; and 

 when the wood has been removed, 

 the formation of new wood takes 

 place immediately beneath the 

 bark. 



