ii8 



MORPHOLOGY OF TISSUES. 



pits (ducts) behave in reference to the side-walls exactly like tracheides (Fig. 25, p. 26). 

 The separate elements of the vessels of Ferns composed of prosenchymatous cells (Fig. 

 ay, p. 27) may be correctly designated tracheides. 



The Prosenchymatous cell-forms of the xylem are always fusiform, very thick-walled 

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

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

 contain starch. Next to the middle lamella of their partition-walls there often lies 

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

 solution, and resembles many bast-fibres. These cells are generally much lunger 

 than the vascular forms. Sanio distinguishes two forms ;— the simple {Hbriform) and 



3=f?^^ 



Fig. 9S. — 1 ransverse section through the phloem of 

 a fibro-vascular bundle in the stem of Cucurbita Pepo 

 ( X 550) ; si the septa of the young sieve-tubes with areolae, 

 the sieve-pores being not yet developed ; / / phloem- 

 parenchyma ; cc cambium. The bast-fibres are here want- 

 ing, the whole of the phloem consisting of soft bast. 

 (Respecting the sieve-tubes see Fig. 74.) 



FlC. 99 —Longitudinal section through the phloem of a 

 fibro-vascular bundle of Ciimrbita Pepo ; three sieve-tubes are 

 visible, whose septa ^ q are not yet perforated ; the protoplasm 

 si and ps contained in the sieve-tubes is contracted ; si a 

 young sieve-plate in the side-wall ; at x and / sieve-pores will be 

 formed later; z narrow parenchymatous cells between the 

 sieve-tubes. 



the septate 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 prosenchymatous 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 libriform 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 by transverse division of the cambium-cells before their thicken- 

 ing commences. The sister-cells show this origin chiefly by the mode in which they are 



