lOO MORPHOLOGY OF TISSUES. 



of the vessels of F>rns composed of prosenchymatous cells (Fig. 29) may be correctly 

 designated tracheides. 



The Fibrous cell-forms of the xylem are always prosenchymatous and fusiform, very 

 thick in comparison with their diameter, with usually simple, but sometimes bordered pits, 

 the pits small ; always without a spiral band ; and during the repose of vegetation 

 containing starch. Next to the middle lamella of their partition-walls there more often 

 lies an unlignified gelatinous thickening-mass which is coloured red-violet by Schultz's 

 solution, resembling many bast-fibres ; these cells are generally much longer than the 

 vascular forms. Sanio distinguishes here also two forms ;— the simple (Libriform), and 

 the partitioned fibres ; the latter are distinguished from the former by their cavity 

 being partitioned by several thin septa, while the common wall of the whole fibre is 

 thick. These fibre-like cell-forms are found in the wood of dicotyledonous trees and 

 shrubs in the most various intermixture with the vascular elements and the other 

 forms to be named immediately. Whether wood-fibres occur in Cryptogams is at 

 least doubtful. 



The Parenchymatous cell-forms of the xylem are widely distributed, and especially 

 abundant when the woody substance of the fibro-vascular bundles attains a considerable 

 thickness. They arise, according to Sanio, in the wood of Dicotyledons and Gymno- 

 sperms by transverse division of the cambium-cells before their thickening commences. 

 The sister-cells show this origin chiefly by the mode in which they are arranged; when 

 completely developed they are thin-walled, with simple closed pits. Their contents 

 in winter consists of starch, often associated with chlorophyll, tannin, and crystals of 

 calcium oxalate. It also happens sometimes that the cambium-cells on the xylem-side 

 of the bundle become transformed without transverse division into parenchymatous, thin- 

 walled, simply pitted, conducting, elongated cells, which must also be considered as paren- 

 chymatous forms of wood-cells'. To this last type are also to be referred the parenchy- 

 matous elements in the xylem portion of the closed fibro-vascular bundles of Monocotyle- 

 dons and Cryptogams ; but these thin-walled, mostly elongated, conducting cells do not 

 in this case originate in the cambium (since this, according to the terms in customary use, 

 is absent from the closed bundles), but immediately from the procambium of the bundle 

 (Fig. 83, near S). Sometimes the wood-parenchyma resulting from the cambium of 

 Dicotyledons (parenchyma of the xylem portion) attains a stronger development, while 

 only a few vessels and tracheides are formed : this occurs in the thick napiform roots 

 of the radish, carrot, beet, and dahlia, and in potato-tubers. The apparent pith of these 

 organs corresponds, in its origin, to the woody substance of a dicotyledonous tree ; 

 but the elements- of the xylem are not, or only slightly, lignified ; the succulent 

 contents and the thin soft cell-walls scarcely give this xylem the appearance of an 

 analogue of the ordinary woody substance, although there can be no doubt about 

 this analogy. 



The Layers of Phloem of the fibro-vascular bundle show, when fully developed, 

 similar celUforms to the xylem portion ; the sieve-tubes correspond to the vessels, the 

 bast-parenchyma to the wood-parenchyma, the true bast-cells to the woody fibres. 

 In the phloem as in the xylem, the different cell-forms may arise in the most various 

 intermixture, sometimes in alternate layers, sometines irregularly. A very general 

 cell-form in the phloem is the Cambiform, consisting of narrow, usually elongated, 

 thin-walled, succulent cells which sometimes appear, in very thin bundles, to form 

 the only constituent of the phloem. When this last is perfectly developed, regular 

 latticed cells arise, which are not always to be easily distinguished from true sieve-tubes ; 

 the formation of the latter has been already explained in Figs. 23 and 24. The perfora- 

 tion of their older sieve-discs, especiahy on the septa, which may lie obliquely or 

 transversely to the longitudinal rows of cells, can be easily proved by laying thin 

 sections in concentrated sulphuric acid, especially if the preparation is saturated 



* Sanio applies to these cells the term ' Ersatzzellen.' 



