FORM OF THE PLANT-CELL. 55 



Frequently, in the later-formed vascular cells, the septum is broken 

 down in such a manner that it remains in a circle as a small edge. This 

 septum is seldom entirely horizontal, but ordinarily somewhat inclined 

 from the axis of the plant towards the radius, very seldom towards the 

 periphery. Such cavities in the septum may be frequently seen by a 

 longitudinal section of a vessel. Treviranus * first remarked this, but 

 knew not what to make of it. Meyen f gave a very unsatisfactory expla- 

 nation of it. This breaking up of the septum only occurs where there is 

 a kind of resistance on the part of the septum itself. Where this ten- 

 dency is very strong, instead of a single cavity many are formed, and 

 the septum acquires a regular ladder-like aspect ; a fact first announced 

 by Mohl. J Examples may be seen in the Beech, in the roots of Palms, 

 in Arundo Donax, &c. Again, the tendency may be so strong, that the 

 cells may be regarded as lying on one another, and there is formed upon 

 the septum, according to the nature of the cell, spiral fibres or pores. 



That the completely developed vessels contain only air, is a fact that 

 can be ascertained by the naked eye. Sometimes, in old age, abnormal 

 fluids will be found in them, and cells are developed in the vessels. Cells 

 are found in the old porous vessels of the Oak and Elm, and I have found 

 them frequently in the spiral vessels of the stems of Scitaminece, as 

 Canna and Hedychium. Such cells do not appear to me to originate in 

 the vessels, but arise from neighbouring cells being pressed into the 

 vessel between the spiral fibres. The cell thus pressed into the vessel 

 originates the new cells. The so-called monilifonn vessels are not 

 formed in a different manner from others. 



26. D. VASCULAR BUNDLES. This term is applied to a mass 

 of lengthened cells, of which a part have been changed into vessels, 

 and which is more or less clearly distinguished from the parenchyma, 

 which they penetrate in longer or shorter masses. They are 

 either, 



a. Simultaneous vascular bundles, when all parts of the bundle 

 originate and are developed at the same time, as seen in the 

 Cryptogamia. 



b. Successive vascular bundles, when the individual parts of the 

 bundle, and especially in the stem, arise and are developed from 

 within outwards. At first they consist of a delicate cellular tissue, 

 filled with an opaque fluid (cambium), which, whilst internally it 

 forms lengthened cells and vessels, goes on increasing externally. 

 These may be divided into, 



1. Definite or closed bundles. In these the growth of the bundles 

 only continues for a short time ; they then become surrounded by a 

 sharply defined cellular tissue, and are incapable of further develop- 

 ment. Ordinarily the vessels lie in a line, or are formed from 

 within outwards ; externally, or on both sides the line, are seen a 

 pair of large porous vessels, and the whole is surrounded and mixed 

 with lengthened thick-walled parenchyma, which distinguish the 



* Vom inwendigen Bau der Gewiichse, Gottingen, 1806, tab. i. fig. 10. b. 

 f Phytotomie, S. 264. 



J DC Palmarum Structure, tab. w. figs. 13, M, 15. 



E 4 



