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. 
27, 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 longer 
than the vascular forms. Sanio distinguishes two forms ;— the simple {libriform) and 
Fig. 98. — Transverse section through the phloem of 
a fibro-vascular bundle in the stem of Cttcurbita Pepo 
(X 550) ; si the septa of the young sieve-tubes with areolae, 
the sieve-pores being not yet developed ; / / phloem- 
parenchyma ; c c cambium. The bast-fibres are here want- 
ing, the whole of the phloem consisting of soft bast. 
(Respecting the sieve-tubes see Fig. 74.) 
Fig. 99. — Longitudinal section through the phloem of a 
fibro-vascular bundle of Cttcurbita Pepo ; three sieve-tubes are 
visible, whose septa q 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 
