72 
MORPHOLOGY OF TISSUES. 
be consulted further on the growth of Acrochcsticum pul'vereum, Stypopodium atomarium, 
Delesseria Hypoglossum, and the leaves of Mosses ^. 
Sect. 13. Formation of the Common Wall of Cells combined into a 
Tissue^. — If the cell-wall between two adjoining cells is thin, it appears, even 
when very highly magnified, single ; and some- 
times this is also the case when it has already 
attained a considerable thickness, as in succulent 
parenchymatous cells. Usually it is only when the 
wall has become moderately thick that it can be 
seen that the one side of the partition-wall belongs 
to one, the other to the adjoining cell. If strati- 
fication and differentiation into layers occur in a 
sufiQciently thickened w^all between two cells, a 
middle lamella always becomes discernible (Fig. 
57, 7Ji), on either side of which the cellulose is 
superposed in the form of layers and shells gene- 
rally symmetrically distributed, so that those on 
one side appear to belong exclusively to the 
one adjoining cell, those on the other side to the 
other c^ll (Fig. 57, i). The impression may thus 
be given as if the layers which are concentrically 
deposited round each cell-cavity formed the wall 
belonging to it alone, while the middle lamella 
belonged to a common matrix in which the cells 
are imbedded ; or as if it were excreted from the 
neighbouring cells. Both views were actually held 
for a considerable time, and the middle lamella 
was then termed 'Intercellular Substance.' If the 
older fragments of tissue represented in Fig. 57 are 
compared with the younger condition of the same, 
the thought at first suggests itself that the middle 
lamellae may be the original thin walls, on which 
the thickening-layers have been deposited on both 
sides; this view has also found its defenders, 
by whom the middle lamella was distinguished 
as the ' Primary Cell-wall.' The remaining thick- 
ness is then correspondingly described as ' Se- 
is differentiated into two shells, as * Secondary ' and * Tertiary 
Fig. 57.— Transverse section tlirough thick- 
ened cells with evident formation of middle 
lamellae {m) ; z the whole of the cellulose super- 
posed on this middle lamella ; / the cavity of the 
cell, from which the contents have been removed. 
A from the cortical tissue of the stem oi Lycopo- 
duim Cha^nacyparissns ; B wood-cells from the 
inner part of the wood of a young fibro-vascular 
bundle of the sunflower ; C wood of Pimis syl- 
vestris, st a medullary ray (x 800). 
condary,' or 
Cell-wall' 
if it 
On the formation of the cortex of Ceramiaceae see Nägeli, Die neueren Algensysteme 
(Neuenbürg 1847), and Nägeli und Cramer, Pflanzenphysiologische Untersuchungen. 
^ H. V. Mühl, Vermischte Schriften botanischen Inhalts, Tübingen 1845, p. 311 et seq. — ■ 
Ditto, Die a egetabilische Zelle, p. 196. — Wigand, Intercellularsubstanz und Cuticula. Braunschweig 
1850. — Schacht, Lehrbuch der Anatomie und Physiologie der Gewächse, 1856, vol. I. p, 108. — 
Müller, Jahrb. für wiss. Bot. vol. V. 1867, p. 387. — Hofmeister, Lehre von der Pflanzenzelle. Leipzig 
1867, § 31. 
