SCIEXCE-GOSSIP. 



000 



series of crossing lines in different directions in one 

 layer, but they belong to two. This theory was 

 the first attempt to apply mechanico-physical 

 considerations to the explanation of the pheno- 

 mena of organic life. 



Next came Strasburger, following Dippel and 

 others. He throws over altogether Nageli's con- 

 ception of micellae, and to his mind only molecules 

 of cellulose exist. To him the lamellse, seen 

 as stratification, are the expression of intermittent 

 thickening by apposition, thus re-instating Von 

 Mold's theory of cell-wall growth. The outer coat 

 of a lamella differs from the main body of the 

 same lamella, and the place of junction of the 

 lamellae is therefore indicated by difference in the 

 refractive index. Striation he explained by saying 

 that each lamella is not plastered on as a whole, 

 but as a ribbon, or more in one place than in 

 another, the striae being the contact lines. 



A few years ago an important paper on the 

 cell-wall and striation a 



was published by 

 Krabbe, who insists 

 that striae do not 

 correspond to what 

 Strasburger believed, 

 but that the lamella 

 in ordinary cells are 

 plastered on as one 

 sheet, and that the 

 whole appearance of 

 striation is due to 

 subsequent change. 

 Since the publication 

 of this paper, Stras- 

 burger has re-investi- 

 gated the question, 

 and has practically 

 abandoned his theorv 

 of striation, although 

 still maintaining that thickening takes place by 

 apposition, especially in such cases as the bars in 

 tracheides and the spiral thickening in some 

 vessels. 



Apposition explains the growth in thickness of 

 cell-walls, and derives confirmation from the study 

 of the growth of starch grains, which almost 

 certainly takes place in this way. But growth in 

 surface has always been somewhat of a difficulty 

 from the apposition point of view-. This was at 

 first overcome by assuming that the layers were 

 stretched by pressure exerted from within, and that 

 whilst in this stretched condition fresh lamellae 

 were plastered on, and that the optical properties 

 were the expression of tensions. But there are cases 

 where cell-walls grow in surface when there can be 

 no question of stretching, as by reducing artificially 

 the turgidity of cells when the growth in surface is 

 not retarded to such an extent as might be expected. 



Cell-Wall. — a. Bordered pits in section. b, CEdogonium 

 showing thickening ring (dotted lines), c, CEdogonium showing 

 thickening ring stretched out (a a), so intercalating new piece of 

 cell-wall, d. Middle lamella (three linesi, secondary thickening 

 layer and limiting la ver. e, Cell-wall, showing stratification. /, Cell- 

 wall, showing striation. 



The peculiar mode of cell-growth seen in CEdo- 

 gonium, etc., is also a difficulty from the apposition 

 point of view. 



Here a solid ring appears which splits and 

 eventually stretches out and increases the size of 

 the cell-wall. The ring shows no signs of growth 

 by apposition, and Strasburger himself admits 

 that it probably grows by intercalation of fresh 

 material, although not at all according to Xageli's 

 theory. 



Lastlv we come to Weisner, who says that while 

 the cell-wall is growing it always contains proto- 

 plasm, by the activity of which what he calls derma- 

 tosomes, which form ultimately the cell-wall, are 

 secreted. This finds some confirmation from the 

 subsequent changes that cell-walls undergo, e.g., 

 lignification, etc. When the cell-walls are at all 

 thickened in a tissue w T e can see clearly two 

 regions — the party-wall or middle lamella and the 

 rest of the wall (secondary layer) on each side. 

 There can also be 

 distinguished a third 

 portion — the inner 

 limiting layer, or 

 tertiary layer. 



The middle lamella 

 is partly the primary 

 septum common to 

 two cells, and this 

 commonlv, when the 

 secondarv layers are 

 changed, retains a 

 cellulose character. It 

 differs from the sur- 

 rounding wall, in that 

 it is most soluble in 

 Schultz's macerating 

 fluid, and, therefore, 

 gives way first. It 

 also gives way when 

 schizogenous intercellular spaces are formed. 



The secondary layer shows the stratification, and 

 is the layer which principally undergoes secondary 

 changes, lignification, etc. But the growth and 

 thickness of a cell-wall is often not uniform ; 

 thickenings take place locally, sometimes on the 

 outer side, as in pollen-grains, and often when cells 

 are combined into tissues on the inner surface, 

 giving rise to annular, spiral, scalariform, and 

 other thickenings, and to simple and bordered pits. 

 In the latter case the unthickened parts of con- 

 tiguous cells are opposite one another. 



Although cell-walls are always first formed of 

 cellulose (except pectose) they frequently undergo 

 chemical, accompanied by physical, changes. 

 These changes do not generally begin until the 

 cell has acquired its full size. 



With regard to lignification, there is some doubt 

 as to what causes this change, and the term 



