ASCENT OF WATER IN MüNOCOTVLEDONS. 



^31 



The necessity of the assumption that in the vascular bundles of the Monoco- 

 tyledons it is not, or is only in a very subordinate degree, the lignified xylem itself 

 which conveys the ascending current of water, but that rather the lignified scle- 

 renchymatous sheaths play the main part in the process, is very evident on 

 regarding the transverse section of the vascular bundle of a graminaceous plant 

 (as Fig. 195). Here the area of the lignified xylem is only a fraction of that 

 of the sclerenchymatous sheath. Besides, the fibrous elements of the latter in 

 such cases agree in all essential points with the proper wood fibres of the 

 secondary dicotyledonous wood. In other cases, as in the scapes of species 



Fig. 193.— Part of the transverse section of the stem of 

 a Dracdna, about 13 mm. diameter. The very slender and 

 thin walled proper vascular bundles are surrounded by thick, 

 lignified sheaths of sclerenchyma, which, according to my view, 

 convey the ; 



Fig. 194.— Transverse section of the stem of Cyathta Im- 

 rayaiia (nat. size). All the black streaks (s s') indicate lignified 

 sclerenchyma; the grey streaks and dots vascular bundles (a). 

 (De Bary.) 



of Allium, the hollow cylinder of sclerenchyma is practically only lignified 

 parenchyma; but here, as likewise in the Piperacea;, it is not clear in which 

 forms of tissue the current of water ascends, if not in the layers of scleren- 

 chyma. Even in Dicotyledons it often happens that lignified strands or layers of 

 sclerenchyma run through the shoot-axes, in addition to feeble vascular bundles 

 deficient in wood. 



Hitherto all these sclerenchymatous structures have been considered only 

 as means for promoting elasticity and rigidity, as they and the true wood 

 in fact are. I conclude, on the other hand, that all layers of lignified tissue which 

 may be followed continuously from the root through the stem into the transpiring 



