7^ 



MORPHOLOGy OF TISSUES. 



be consulted further on the growth of Jicrochaticum pul'vereum, Stypopodium atomnrium^ 

 Delesseria Hypoglossunt, and the leaves of Mosses ^ 



Sect. 13 

 Tissued— If 



Formation of the Common Wall of Cells combined into a 



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 

 sufficiently thickened wall between two cells, a 

 middle lamella always becomes discernible (Fig. 

 57, m), 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 cell (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- 

 condary,' or if it is differentiated into two shells, as * Secondary ' and * Tertiary 

 Cell-wall.' 



Fig. 57.— Transverse section through thick- 

 ened cells with evident formation of middle 

 lamellae (w) ; i 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- 

 dium ChamiEcyparissiis ; B wood-cells from the 

 inner part of the wood of a young fibro-vascular 

 bundle of the sunflower ; C wood of Finns syl- 

 ■vestris, si a medullary ray (x 800). 



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

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



^ H. V. Mohl, Vermischte Schriften botanischen Inhalts. Tubingen 1845, p. 311 et seq. — 

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

 1850. — Schacht, Lehrbuch der Anatomic und Physiologic der Gcwachse, 1856, vol. I. p. 108. — 

 Miiller, Jahrb. fiir wiss. Bot. vol. V. 1867, p. 387. — Hofmeister, Lehre von der Pflanzenzelle. Leipzig 

 1867, § 31. 



