88 



ON VASCULAR STRUCTURE. 



is quite evident from the fact that twining plants may encircle it for many years 

 without compressing it ; but this is begging the question ; for if the stem be fully 

 developed, as at first referred to, no further increase of the tree is expected. The truth 

 seems to be, that endogons cease to grow at their lower part, whilst growth proceeds 

 above ; and thereby the cuticle may have attained to a maximum of extension near to 

 the base, whilst it may be comparatively undeveloped above. 



Schkiden explains the peculiarities of the cuticle of endogenous stems, as distin- 

 guished from that of exogenous ones, by employing the term " limited growth" to the 

 former, and "unlimited growth" to the latter; and explains them by stating, that in 

 the former, after a certain period, the production of the fast-growing thin walled- cells 

 of the cuticle ceases, and the partitions become thicker ; whilst in the latter, the cells 

 are continually reproduced throughout the whole period of growth of the plant. This 

 seems to be rather a statement of the facts than an explanation of them. 



It is the fashion to state that endogens have no bark, since none is separable from 

 the wood, and that the cuticle is simply the hardened exposed cells of the stem, with 

 the ends of bundles of woody fibre intermixed. If analogies are truly founded upon 

 function, and not upon structure, we must admit that there is a cuticle or external 

 protective covering to endogenous stems. 



Vascular Structure. This is a mixture of woody fibre and bothrenchym, with 

 the addition of spiral ducts or spiral vessels. 



If we examine a transverse section of a cane, we do 

 not find a central pith, with wood arranged in layers 

 around it, but a surface marked by fourteen cut ends of 

 round bundles of vascular tissue, set in a cellular matrix. 



f-oi// "K ] I >C V ; N' ^ a ^ so on ma ki n g a perpendicular section, we notice that 



v^-. li/T^ /( /vxvfi fO *ke surface of the section presents a number of perpendicu- 

 lar defined lines, which may be torn out, and a series of 

 intervening connecting substances. The distinguishing 

 peculiarity of endogens is the arrangement of the woody 

 fibre. 



The general direction of the woody fibre is clearly 

 from above downwards ; but it is highly probable that it 

 does not descend in straight lines, and that when the tree 

 has attained a tolerable height the wood does not descend 

 directly to the root. As the structure is not indigenous 



145, showing the arciform to climes where scientific men abound, the observers have 

 arrangement of the bundles been few ; and as the subject is an intricate one, it has not. 

 of woody fibre m endogens . ,. ,.-,, - r i i -, i t 



(as the Palm), as they pass as yet, been fully elucidated. Mohl is the best authority, 

 from the series of interfoliar and he affirms that $& bundles of fibres descend from the 

 organs at the head of the 

 stem, and their parallel leaves in arcs, which direct then- convexities towards the 



sM^of The hark n tlie inner centre of the stem. Thus the fibre, in its descent, first 



a ft, fully developed part of passes towards the centre, and thence towards the circum- 



to d, 8t Strices of leaves, with ferer ice, until it reaches the bark, or nearly so, when it 



their vascular bundles. passes down . in a more direct manner towards the root. 



the'biS dl6S Pr C <iing lr m Each fibre will therefore somewhat represent a hedge- 



/, the latest-formed leaves, hook with a long handle that is, have the form of an arc 



After Schleiden. . , .. , , 



above, and a straight line below. The centre of each arc 



will not correspond with the central point in the height of the stem, but will be distant 



