222 A SKETCH OF 



it has been observed, that the branches of Cactus stems, 

 cut off and left forgotten in a corner without further 

 care, far from dying, have frequently grown on and 

 made shoots three feet long or more. De Candolle 

 first found the right path, when he weighed such Cactus 

 shoots which had grown without soil, and found that 

 the plant, though larger, was always lighter, therefore, 

 instead of abstracting anything from the atmosphere, 

 must rather have given up something to it. All the 

 growth takes place, in such cases, at the expense of the 

 nutritive matter previously accumulated in the juicy tissue, 

 and it generally exhausts the plant to such a degree, that 

 it is no longer worth preserving. It is that succulent 

 tissue which enables the Cactus plants, one might com- 

 pare them with the Camels, to provide themselves before- 

 hand with fluid, and thus to brave the rainless season. 

 Their anatomical structure also assists them in this respect, 

 in a peculiar manner. We know, from the experiments of 

 Hales, that plants chiefly evaporate the water they contain, 

 through their leaves, and the Cactus tribe have none. 

 Their stem, too, unlike that of all other plants, is clothed 

 with a peculiar leathery membrane, which wholly prevents 

 evaporation. This membrane is composed of very strange, 

 almost cartilaginous cells, the walls of which are often 

 traversed by elegant little canals. Its thickness varies in 

 different species, and it is thickest, and therefore most 

 impenetrable in Melocacti, which grow in the dryest 

 and hottest regions, while it is least remarkable in the 

 species of Rhipsalis, which are parasites on the trees of 

 the damp Brazilian forests. 



Another striking point about this group, is the formation 

 of an extraordinary quantity of oxalic acid. If this acid 

 were collected in large amount in the plant, it must neces- 



