28 



MORPHOLOGY OF THE CELL. 



peared (c, c, in B), the thickening-ridges have become overarched, growing inwards, so 

 that now only a narrow fissure (<r/, B) remains between its margins ; still further inwards 



the ridge again becomes narrower. The interior cavities of 

 two adjoining vessels are thus united by a number of broad 

 fissures (5, j) ; the frame-work of the ladder is formed of 

 peculiarly-shaped rungs, which may be seen in B at cc in 

 section, at e along the surface. Where the wall of a vessel 

 bounds a parenchyma-cell (E), the scalariform thickening 

 takes place only on the side towards the vessel (g), it is 

 absent from the other side (p). In this case also the thin 

 original wall remains ; it closes the broader exterior space 

 of the bordered fissure-shaped pit. 



The variety in the formation of pits is by no means 

 exhausted by these examples ; but all the processes cannot 

 be described here ; we can only indicate a few. 



We saw in the formation of vessels in the Dahlia (Fig. 

 27) how the pit occupies at first a large round space, while 

 the margins of the overarching thickening enclose a fissure. 

 By a change in this process of growth, the fissure may 

 attain a length much greater than the diameter of the 

 exterior cavity of the pit ; then the pit appears, on a front- 

 view, as a roundish opening, crossed by a fissure (Fig. 28, 

 P). It also sometimes happens that the pit-fissure changes 

 its direction as the thickening increases, and in this case, 

 on a front -view, two fissures crossing one another are 

 perceived (Fig. 30, ^ and B, st). But in order to be cer- 

 tain that this takes place within the layers of the wall of 

 one cell, the cells must be isolated by maceration. Similar 

 appearances are also often presented on a front-view, if the 

 whole partition-wall of two cells is observed from the 

 front. If the fissure runs upwards to the left in the one 

 cell, the corresponding fissure may run upwards to the 

 right on the other side ; on a front-view they then ap- 

 pear crossed ^ 



In cells of tissues the partition- wall is always at first a 

 very thin simple lamella ; as the thickness increases, the 

 thickening-masses always project right and left into the 

 adjoining cell-cavities. Generally the growths right and 

 left of a partition-wall, as we have already seen, corre- 

 spond ; and this occurs most clearly in the formation of 

 pits, as far as the pit-channels of adjoining cells meet 

 one another. But since a cell often adjoins very different 

 contiguous cells on different sides, different sides of the 

 same cell may show different forms of thickening, and 

 especially different formations of pits. The total growth 

 in thickness may also be very different on different sides ; 

 thus, for instance, the epidermis-cells on the outer free 

 wall are mostly very strongly thickened on the inner wall, where they adjoin paren- 

 chyma-cells, being either very thin or corresponding in form to the adjoining cells. 



Fig. 30.— Brown-wailed cells jn the stem of 

 Pteyis aqiuUna ; A a half-cell' isolated and 

 rendered colourless by Schulze's macera- 

 tion ; B a piece more strongly magnified 

 (X550); the fissure-like pits are crossed; 

 2. e. the fissure is twisted as the thickening 

 increases ; at / side-view of a fissure, ap- 

 pearing here as a simple channel, since it 

 shows the narrow diameter. 



Fig. 31. — Transverse section of a bast-cell of 

 the root -tuber oi Dahlia variabilis (x8oo); 

 /the cell-cavity; A' pit-channels which pene- 

 trate the stratification ; sp a crack by which 

 an inner system of layers has become sepa- 

 rated. 



^ A very clear representation of a twisted pit-channel, whose outer and inner fissure (within the 

 same cell-wall) cross, may be seen in Nageli, Berichte der Miinchener Akademie, 1867, vol. v. 

 fig- 45- 



