PARENCHVMA. 



117 



The forms of thin-walled parenchymatous cells are in the main nearly iso-dia- 

 raetric ; but there often occur also elongated-prismatic, spindle-shaped cells, and the 

 like, examples of which, e. g. in the case of the vascular bundles, will be described 

 later; to this category belong also those chlorophyll-containing cells arranged in 

 many leaves perpendicular to the surface, forming the pallisade parenchyma, to be 

 described in Chapter IX. 



As above intimated, very great variety of shape is found among the iso -diametric 

 forms. It is only in definite single cases, e. g. in hypodermal sap-parenchyma, that 

 the cells are of such form that all of them are bounded by flat surfaces and sharp edges, 

 and therefore are in uninterrupted connection with one another. As a rule the surface 

 of the parenchymatous cells is more or less rounded, or bears irregular protuberances, 

 or the protuberances themselves are drawn out into long arms : in this case they are 

 mutually connected only by definite parts of their surface, which vary in size accord- 

 ing to the special form. Between them intercellular spaces are left free. Masses of 

 parenchyma in which the latter (which are then usually filled with air) are developed 

 to a great extent are distinguished as lacunar parenchyma, or, comparing it with a 

 bath-sponge, spongy-parenchyma. Compare Chapters VII and IX. 



The walls of the cells of this category are as a rule cellulose membranes, with 

 ordinary simple pitting. The latter, following the general rule, usually occurs only 

 on the parts of the surface in contact with that of other cells : in cases then where 

 the cells show a decided partial rounding off, and only touch one another with 

 narrowly limited parts of their surface, or only with the ends of protuberances, the 

 pits lie on these spots, and not on the rest of the wall. As regards the surfaces of 

 contact, the same may also occur with dissimilar tissue-elements. When similar cells 

 touch one another by the ends of narrow protuberances, 

 there is often only a single pit on each protuberance ; 

 larger circumscribed surfaces of contact appear as pitted 

 fields on the otherwise smooth wall (Fig. 46). This phe- 

 nomenon, which was known long agoS and which occurs 

 especially often in round-celled chlorophyll-parenchyma 

 of succulent plants, resembles that of the sieve-plates of 

 the sieve-tubes (Chap. V) ; but it is incorrect to place it 

 side by side with this ^ since the characteristic structure 

 of sieve-plates is wanting in the parenchymatous cells, 

 though the pitted fields also in the parenchyma of the leaf 

 of Cycads, specially of Encephalartos, are distinguished 

 from the rest of the wall by brown coloration in Schultze's 

 solution, and deep red coloration in solution of Anilin'. 



Fibrous partial thickenings of the walls are known 

 here and there, e. g. in the form of reticulate or spiral 

 fibres, in the watery hypodermal parenchyma of the leaves 

 of the Pleurothallideae, and in many roots of orchids ; as reticulate fibres in the 



Fig. 46. — A parenchymatous cell 

 from the cotyledon of Phaseolus inul- 

 tiflorus, isolated by maceration; ii 

 the parts of the wall -where it bor- 

 ders on intercellular spaces, / 1 the 

 pitted parts of the surface which bor- " 

 der on neighbouring- cells ; the thin- 

 nest points of the pits are shaded dark 

 {550). From Sachs' Textbook. 



^ See e.g. Schleiden, Grundz. 3 Aiifl. I. p. 245. 



=^ Areschoiig. Botan. Zeitg. 1870, p. 305 ; and Acta Univ. Lund. torn. IV.— Borczow, in Pring- 

 sheim's Jahrb. torn. VII. ^ ICraus, Cycadeenfiedern, I.e. 



