THE VEGETABLE ORGANIZATION. 2\fO 



SO as to form a cylindrical tube of greater or less extent, its 

 central cavity constitutes the internal tubular cavity of the vessel, 

 and the secondary vesicles of its envelope still retaining their 

 spiral arrangement, form the spiral externalcoatingof the vessel. 

 The communication between several of these elongated cells or 

 tubes may afterwards take place, so as to form a continuous 

 vessel of considerable extent in the same manner as in the more 

 simple vessels. These spiral vessels are very readily seen 

 in the leaves of many plants. By carefully breaking a strongly 

 nerved leaf, and gently drawing the parts asunder, the spiral 

 vessels of the nerve will become visible, and the spiral texture 

 may be unrolled to a considerable extent without breaking. 



These several kinds of vessels pass into each other, forming the 

 mixed vessels of Mirbel ; the same vessel being in one part of 

 its extent porous, in another transversely slit, and in a third 

 spiral. The explanation of this modification of vascular de- 

 velopment appears to be that cells and tubes formed under 

 varying circumstances of pressure or other modifying causes, 

 and consequently differing in the arrangement of the particles of 

 which they consist, have become united together so as to form a 

 continuous vessel. 



There are two other kinds of vessels which require to be 

 noticed. These are simple tubes and the proper vessels. The 

 proper vessels of plants are short tubes without pores, and are 

 found in the bark, leaves, and flowers. They contain the peculiar 

 secretions of the plants in which they are found, as the yellow 

 acrid juice of the Celandine Cheledonium majus, the opaque milky 

 juice of the poppies, &c. They are, therefore, elongated cells, 

 appropriated as reservoirs for the proper juices furnished by the 

 individual plants. The simple tubes are vessels which are with- 

 out pores, and appropriated to the circulation of the sap, and 

 hence called sap-vessels. They do not differ in their mode of 

 development from other vessels, but in consequence of the 

 secondary cells of their coats being more closely compacted, no 

 interstitial spaces or pores are visible in their sides. 



In the more simple and elementary forms under which plants 

 present themselves to our notice, the cellular texture alone is found 

 to occur. These plants, the Cellulares, or Cellular plants of De 

 Candolle, constitute one of the primary divisions of the vegetable 

 kingdom. They differ greatly, both in their internal structure and 

 external configuration, in the several organs and parts of which 

 the vegetable frame consists — in their roots, stems, branches, 

 leaves, and parts of fructification, not only from plants of a 

 higher order, but also among themselves. There is little 

 apparent analogy between the rose which delights us with its 

 fragrance, and the noisome /z^w^w^. Nature's scavenger — between 

 the sturdy oak, and the crust which attaches itself to the surface 

 of the solid rock, either as a whole or in any of their parts ; and 

 yet the transition from one of these extremes to the other, through 



