70 



MORPHOLOGY OF TISSUES. 



Nageli and Schvvendcner (Das Mikroskop, II. 563 et seq.) may be consulted further on 

 the growth of Acrochcetlcum pul'vereum, Stypopodium atomarium, Delesseria, Hypoglossumy 

 and the leaves of Mosses 1. 



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, as a simple lamella ; and sometimes this is also the 



case when it has already attained a considerable 

 thickness (in succulent parenchyma-cells). Usually 

 it is only when the wall has attained some thick- 

 ness that it can be seen that the one side of the 

 partition-wall belongs to one, the other to the other 

 adjoining cell. If stratification and differentiation 

 into layers occur in a sufficiently thickened wall 

 between two tissue-cells, a middle lamella always 

 becomes discernible (Fig. 57, 7n), on which, right 

 and left, the remaining cell-substance is superposed 

 in the form of layers and shells, generally sym- 

 metrically distributed, so that those on one side 

 appear to belong exclusively to the one adjoining 

 cell, those on the other side to the other (Fig. 57, i). 

 The impression may thus be given to the observer 

 as if the layers which are concentrically deposited 

 around each cell-cavity formed the wall belong- 

 ing to it alone, while the middle lamella belonged 

 to a common matrix in which the cells are im- 

 bedded ; or as if it were excreted from the neigh- 

 bouring 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 com- 

 pared with the younger condition of the same, the 

 thought at first suggests itself that the middle la- 

 mellae may be the originally thin walls, on which 

 the thickening-layers have been deposited on both 

 sides inward by apposition ; this view has also found 

 its defenders, by whom the middle lamell'a is dis- 

 tinguished as the Primary Cell-wall. The remain- 

 ing thickness is then correspondingly described 



Fig. 57.— Transverse section through thicken- 

 ed cells with evident formation of central lamellse 

 (w) ; i is always the whole of the superposed 

 cell-substance ; / the cavity of the cell, from 

 which the contents have been removed. A from 

 the cortical tissue of the stem of Lycopodhim 

 chajnacypai-issKs ; 5 wood-cells from the inner 

 part of the wood of a young fibro-vascular bundle 

 of the sunflower; C wood of Piniis sylvestris, 

 St a medullary ray (xSoo). 



as secondary ; or if it is differentiated into 

 cell- wall. 



two shells, as secondary and tertiary 



^ On the formation of the cortex of Ceramiaceoe, see Nageli, Die neueren Algensysteme 

 (Neuenburg 1847), and Niigeli und Cramer, Pflanzenphysiologische Untersuchungen. 



2 H. V. Mohl, Vermischte Schriften botanischen Inhalts. Tubingen 1845, p. 314 et seq. — H. v. 

 Mohl, Die vegetabilische Zelle, p. 196. — Wigand, Intercellularsubstanz und Cuticula. Braunschweig 

 1850. — Schacht, Lehrbuch der Anatomic und Physiologic der Gewachsc, I. p. 108, 1856. — Miiller, 

 Jahrb. fiir wiss. Bot. V. p. 387, 1867. — Hofmeister, Lchre von der Pflanzenzelle. Leipzig 1867, § 31. 



