140 



LECTURE VIII. 



even when each of them 



By the presence of these two groups ot tissue, 

 only asserts itself by a few characteristic elements in the transverse section, 

 the vascular bundles may always be distinguished from the strand-likp structures, 



occurring frequently enough elsewhere, which 

 run in the fundamental tissue of many shoot- 

 axes and leaf-stalks in a similar manner to the 

 vascular bundles ; but which are essentially dif- 

 ferent from these, being merely special forms 

 of fundamental-tissue. As the true nature of 

 leaves and roots through the whole series of the 

 vascular plants, though the structural relations 

 are the most different possible, is always to 

 be recognised; so also the vascular bundles 

 of all vascular plants, even when they depart 

 far from the typical structure, as in many 

 water-plants and parasites, yet always appear 

 as tissues of essentially the same nature. I 

 interpolate this remark, because it has recently 

 been attempted to confound these character- 

 istic and constant constituents of the ana- 

 tomical structure with other strand-like ar- 

 rangements of tissue, which may be present 

 or be wanting according to purely biological 

 requirements ; and this to the great injury of Phytotomy, which only loses thereby 

 in clearness and scientific depth. One might just as well name any given filiform 

 organ a root, or any flat structure a leaf, as place the vascular bundles on a 

 level with other haphazard strand-like masses of tissue'. 



Fig. 147.— Parts of sieve-tubes where the segments 

 unite, showing the perforation of the septa after solution 

 of the cell-wall by sulphuric acid. W and B from the 

 petiole oiCucurbiia; C from the stem oi Dahlia. In A 

 the cell-wall A. h' is not yet dissolved, s slimy contents ; 

 o and u accumulation at the upper and xmder side of the 

 septum y p the threads of slimy substance which connect 

 these accumulations, and pass through the pores of the 

 sieve-plate. 



' The attempts repeatedly made of late to place the vascular bundles, as a subordinate form of 

 tissue, approximately on an equal footing with mere sclerenchyma strands, only shows how little the 

 younger botanists have succeeded in comprehending the province of vegetable anatomy, cultivated 

 with such great results by Mohl, Nageli, and De Bary. 



