30 



MORPHOLOGV OF THE CELL. 



and the less dense ones very watery, as is the case with some wood-cells {e. g. Pinus 

 syl'vestris), the striation becomes more evident through desiccation, because then the 

 dense layers stand forwards, while the less dense ones give way. 



The systems of striation and stratification of a cell-wall intersect one another, like 

 the cleavage-planes of a crystal splitting in three directions. But since the striations 

 and laminae consist of lamellae of a measurable thickness, composed of alternately denser 

 and less dense substance, the cell-wall appears to be composed of parallelopipedal pieces, 

 distinguished by their content of water. If we for a moment disregard the stratifica- 

 tion, and assume that we have two intersecting systems of striation, then, where two 

 dense striae intersect, the densest or least watery places are always to be found ; where 

 two dense ones intersect, the least dense or most watery ; and where places of greater 

 and less density intersect, areolae of intermediate density are formed. The intersections 

 of the striae must form prisms which stand vertically or obliquely upon the surface of 

 the cell-wall ; if the concentric stratification is very strongly developed, every one of 

 these prisms must become decomposed into denser and less dense 

 sections lying in radial order behind one another ; if the concentric 

 lamination is feebly developed, the prismatic structure may sometimes 

 appear very clearly ; the peculiar internal structure of the exospores 

 of Rhizocarpeae, and the yet more various structure of the extine 

 of many pollen-grains, may be resolved into a further development 

 of this kind of process ; but our space does not permit us to 

 pursue this in detail. The lamellae which appear externally as 

 striation may possess the form of closed rings ; i. e. may be similar 

 to thin sections of the cell, or may run in a spiral manner round 

 the axis of the cell. A distinction must accordingly be drawn 

 between annular and spiral striation ; it is often, however, very 

 difficult to decide which of the two is present ; sometimes both 

 are developed at different parts of the same cell-wall. Sometimes 

 one system of striation is very obscure, the other more strongly 

 marked ; or one system may be the better developed in one 

 layer of the cell-wall, the other system in another layer ; and 

 this is genetically connected with the above-mentioned twisting 

 of the pit-fissures. The striation is mostly clearest in cells with 

 broad uniform thickening-surfaces (as Valonia utricularis, hairs of 

 Opuntia, pith-cells of the root-tubers of the Dahlia, in the latter 

 case remarkably plain) ; but it may also be recognised when the 

 sculpture of the cell-wall is complicated ; e. g. in the walls of very 

 wide vessels of Cucurbita Pepo, provided with densely crowded small 

 bordered pits (after Schulze's maceration, especially in vessels of the 

 root, very clear crossed spiral-striation). The striation may itself 

 give occasion to diff"erences of level ; sometimes the denser lamellae 

 project a little on the inner side of the cell-wall (Fig. 34, B) ; or single denser lamellae 

 of one system of striation alone become prominent ; thus, for instance, a fine spiral 

 band makes its appearance on the inner sides of the wood-cells of the yew, which is not 

 unfrequently crossed by one running in the opposite direction. When elongated 

 fissure-like pits are arranged in a spiral line on the cell-wall, a system of striation is 

 generally found in a corresponding direction. 



This slight sketch must suffice to introduce the beginner to the nature of stratifi- 

 cation and striation, and to their relation to the sculpture of the cell-wall ; further 

 detail would exceed the limits of this book L 



Fig. 33.— a cell beneath the 

 epidermis of the stem of 

 Pier is aquiluia, isolated by 

 Schulze's maceration. The 

 wall is seen in optical longi- 

 tudinal section; it shows an 

 innermost very dense layer, 

 a central less dense layer 

 (to the right below is the 

 dark striation) enclosed by 

 two denser layers ; these lay- 

 ers are penetrated by pit- 

 channels, which are seen on 

 the hinder wall in transverse 

 section. 



' The striation may easily be seen, even with slight magnifying power, in the large pith-cells of 

 the rf)ol-tiibers of Dahlia, in the hairs of Opimtia, in Valonia vtricidaris ; but only by very high 



