172 PLANT ANATOMY. 
cells, bast and wood-cells (Fig. 136), sieve-tubes (Figs. 147 and 
148), furthermore sickle-shaped, and S- or U-shaped cells. 
Mutual pressure also causes manifold differences of form.. 
Thus from roundish cells (Fig. 80), polyhedral (Fig. 81), and 
more or less rectilineal forms are produced. It is only in very 
delicate and soft a. ea part of fruits, medulla, leaf- 
cells), and in those pl ere the membrane reaches the 
outer air (outer wall of the epidermis and bordering membranes 
of the intercellular spaces), that the spherical outlines remain 
preserved; in firm and hard tissues (wood, Fig. 180, bast 
groups, Figs. 111, 112, 113) all cells are seen to possess, on a 
transverse section, more or less flattened forms with a rectilineal © 
border. 
There are ordinarily distinguished, according to the scheme 
first proposed by Link: 
1. Parenchyma,’ thin-walled, mostly roundish-polyhedral 
and isodiametric cells: cells of the fundamental tissue, of the: 
medulla, of the fleshy part of fruits, merenchyma‘® of the leaves, 
and when extended in a palisade-like manner, palisade-cells * 
(Figs. 80, 81, 85, 108, 127, 128, 129). 
2. Prosenchyma,* consisting of thick-walled, more or less. 
elongated, spindle-shaped cells, with the ends wedged into each 
other: wood-cells, bast-cells (Figs. 139, 136, 110, 111). 
Striking and convenient as the discrimination between pros- 
enchyma and parenchyma appears, yet it does not admit of 
sharp application. 
Fungi and lichens are composed of thread-shaped cells, 
hyphe,* which continue to grow at the ends and mostly divide 
and branch by transverse walls (Fig. 82). They are not only 
‘“Grundlehren der Anatomie und Physiologie der Pflanzen.” Gdt- 
tingen, 1807. 
? Ilapad beside, thereon, and eyyvua that which is poured in, the 
cells considered as standing upon each other. 
* An expression introduced by Meyen (‘‘ Phytonomie,” Berlin, 1830). 
4 Palus, i, masc., a stake, 
* IIpos towards, between, and eyyvuc (eee above), the cells consid-- 
ered as inserted between each other. 
5 ‘Y'o7 the tissue. 
