292 ON THE PROGRESSIVE DEVELOPMENT OF 



be a greater tendency to form the hexagonal cellular tissue, but 

 if placed in a linear arrangement, and subjected at the same 

 time to lateral pressure, the cells will tend to become elongated, 

 and not yielding laterally, the filmy envelopes will give way at 

 their points of contact, and a communication be thus formed 

 from one cell to another, constituting at length a continuous tube. 



Let us now examine the several varieties of vessels with the view 

 of ascertaining how far the opinions here advanced are in accord- 

 ance with the more evident phenomena presented by the vegetable 

 organization. At the points of union between the stem and the 

 root, and the stem and the branches, are found vessels of a monili- 

 form or beaded configuration. They are apparently jointed, and, 

 according to some botanists, are traversed interiorly by transverse 

 partitions. Their external coats, as well as their internal par- 

 titions, are formed of areolar cellular tissue ; that is, of cellular 

 tissue in which (probably from the secondary cells of which it is 

 composed not approximating very closely) interstitial spaces are 

 left. They, therefore, present the appearance of being per- 

 forated. The slightest inspection i§ sufficient to prove that these 

 beaded vessels are nothing more than cells arranged in a linear 

 series, and which, in consequence of the interstitial character of 

 their coats, have become united together, so as to form an im- 

 perfect tube or vessel, the internal partitions remaining in some 

 instances, and disappearing in others, according to the progress 

 made in the development of the true vascular structure. 



In the woody layers of the root, stem, and branches, vessels 

 nearly allied to the preceding are found to occur. These are 

 the punctuated vessels of some botanists, the porous vessels of 

 M. Mirbel. They appear to be continuous tubes, having their 

 sides perforated by pores, which are arranged in transverse lines 

 around the tube. We may readily conceive the beaded or 

 jointed vessel when subjected to considerable lateral pressure, 

 such as must take place in the parts of the plant where these 

 punctuated tubes are found losing the jointed appearance, and 

 having its internal partitions removed. When the pores which 

 are found in the sides of these vessels run into each other, 

 forming apparent transverse slits, the tubes are called false 

 spirals, to distinguish them from the true spiral vessels. The for- 

 mation of these last, the true spirals, appears to be somewhat dif- 

 ferent from the preceding, and to arise from the development of 

 the secondary vesicles constituting the envelope of what may be 

 termed the primary cells. M. Raspail is of opinion that these 

 secondary vesicles are so arranged in the filmy tissue of the en- 

 velope of the primary vesicle as to pass round the cell in a spiral 

 direction, instead of in parallel rows or rings. Now if a cell of this 

 description be gradually elongated, and increase in its progressive 

 development according to the principles before laid down, 

 a tube or vessel presenting precisely the characters of the spiral 

 will result. The whole primary cell is developed and elongated 



