SECT. xiv. CHEMICAL ELEMENTS. 413 



and the cells containing them are microscopic objects, 

 and the solid particles carried by the liquid, which 

 afford the means of tracing its course, are not more 

 than between the three and the five thousandth part 

 of an inch in diameter. M. Schleiden ascribes the 

 motion to changes in the form of the cells produced 

 by an internal vital action, while Professor Karsten 

 believes, from observations he made on the rotation of 

 liquids in the hairs of the common nettle, that it is a 

 phenomenon of diffusion, depending upon the chemical 

 changes taking place in the cells of the hairs indepen- 

 dent of any contractibility, not referable to them. 



The whole of the tissues that exist in a well-grown 

 tree are not to be met with in each of the numerous 

 woody and herbaceous plants of the first class ; some 

 may be wanting, and those that do exist may be, and 

 generally are, much modified both in form and size. 

 All the trees in the temperate zone, and most of those 

 in the tropics, belong to the class of Exogens ; but 

 the annual rings of wood are less distinct in the latter, 

 the periods of repose and activity depending upon the 

 dry and wet seasons not being so decided as our win- 

 ter and summer. The leaves of tropical plants have a 

 thicker skin than in colder climates, to defend them 

 from an ardent sun. The structure of herbaceous plants 

 in all countries is lax and juicy, they have abundance of 

 pith, large medullary rays, and zones of fibro-vascular 

 tubes, which separate the pith from the bark. In 

 fact, each herbaceous and ligneous family has a struc- 

 ture and properties peculiar to itself; but although 

 there is almost an infinite diversity of form and charac- 

 ter, the general type of the class may be traced in all. 



Vegetable matter consists of carbon, hydrogen, oxy- 

 gen, and nitrogen, yet no plant can combine these 

 simple elements into organic substances ; they imbibe 

 them by their roots and leaves under the form of car- 



