THE CELL- WALL. 



27 



cell, the corresponding fissure may incline to the right in the other ; viewed one over 

 the other they then appear crossed ^. 



In cells which form a tissue, the partition-wall is always at first a very thin lamella; 

 as the thickness increases, the thickening-masses project into the adjoining cell-cavities. 

 Generally, as we have already seen, the thickenings on either side of a partition-wall 

 correspond; and this is very evident in the formation of pits, inasmuch as the pit- 

 canals of adjoining cells meet one another. But since a cell often adjoins cells of 

 a very different character, different sides of the same cell may show different forms 

 of thickening and different descriptions of pits. The 

 total growth in thickness may also be very different 

 on different sides ; thus, for instance, epidermal cells 

 are mostly strongly thickened on the outer exposed 

 wall (cuticle); the inner wall, where they adjoin 

 parenchymatous cells, being either very thin or cor- 

 responding in form to that of the adjoining cells. 



The correspondence in the growth of the thick- 

 enings is less evident when they have a distinctly 

 spiral structure, or when they occur in the form 

 of strong spiral bands, as in spiral vessels; if, in 

 this case, in each adjoining cell one or more spiral 

 bands wind in the same direction, they must neces- 

 sarily cross on the common partition-wall. 



(c) Stratification and Striation of the Cell-wall^. 

 When the cell-walls have attained a certain thick- 

 ness and extent of surface, stratification and stria- 

 tion become more or less evident. In consequence 

 of stratification the cell-wall appears as if composed 

 of very thin membranes enclosed one within another 

 and fitting very closely together ; the stratification 

 is seen both in the transverse and the longitudinal 

 section of the cell-wall. The striation is generally 

 to be seen most plainly from the front ; it may be 

 observed in the form of two systems of lines (some- 

 times apparently more) marked on the surface. The 

 one system, consisting of parallel striae, is always 

 intersected by the other system which also consists 

 of parallel striae. A closer investigation shows that 

 the appearance of striation does not belong to the 

 surface only or to one layer of the cell-wall, but 

 that the striation rather penetrates the whole thick- 

 ness of the cell-wall, and that the striae are due to 

 layers which intersect the surface obliquely, and all 

 other layers concentric with it. If the striation is 

 very strongly marked, and nearly parallel to the 

 longer axis of the cell, it may be recognised in a 

 transverse section in the form of lines crossing the 



concentric layers ; in a longitudinal section only those systems of striation are easily 

 seen which, viewed on the surface, cut the longer axis nearly at right angles. 



Fig. 27.— Vessel from the underground stem 

 of Pteris aqiiiliita thickened in a scalariform 

 manner ; A half of a vessel, isolated by Schulse's 

 maceration ; H — D sections obtained from pieces 

 of the stem hardened in absolute alcohol ; B after 

 a very clean section, represented in a partly dia- 

 grammatic manner ; to the right, front view of 

 the wall of the vessels from within ; c c vertical 

 section of the same ; C front view of the young 

 wall of a vessel ; D its vertical section ; E place 

 where a vessel adjoins a parenchymatous cell, in 

 section vertical to the thickening-ridges of the 

 vessel (X 800). 



^ A representation of a twisted pit-canal, whose outer and inner fissure (within the same cell- 

 wall) cross, may be seen in Nageli, Berichte der Miinchener Akademie, 1867 (July 9), t. v. fig. 45. 



"^ H. von Mohl, Bot. Zeitg. 1858, pp. i, 9, — Nageli, Ueber den inneren Bau der vegetabilischen 

 Zellenmembran, in the Sitzungsberichte der Miinchener Akad. der Wissenschaften, 1864, May and 

 Jul^. — Hofmeister, Lehre von der Pflanzenzelle, p. 197. f 



