MECHANICAL CONSTRUCTION OF PLANT-BODY 141 



them together with lime as in Halimeda. These are to be regarded as con- 

 cessions to the mechanical imperfection of the non-septate construction. 



Cases of non-septate tissues exist in the body of some of the higher plants, 

 but as they are embedded in other tissues they are not exposed to mechanical 

 demands. Examples are seen in the latex-cells and vessels, as in the 

 Euphorbiaceae and Cichoriaceae ; and in the young embryo-sacs of the 

 Flowering Plants. 



All ordinary plants of large size are septate. This is specially 

 necessary where they live exposed in the air, and are thus subjected 

 to greater strains than if floating in water. In the embryonic region 

 the cells are seen to divide into equal parts, the newer cell-wall 



FIG. 100. 



Diagram illustrating the plan of arrangement of cell-walls in the apex of the stem 

 of an Angiosperm. XX = axis of construction. EE= external surface. PP=pri- 

 clinal curves. AA =anticlinal curves. (After Sachs.) 



being inserted at right angles on the older walls. This leads to a 

 cell-net the exact detail of which depends upon the external form of 

 the part (Fig. 100). The disposition of the walls is from the first such 

 as to give added mechanical strength, but in the young cell the walls are 

 extremely thin, and are composed of pliant material. In the young 

 shoot the mechanical strength is almost entirely dependent on the 

 turgor of the individual cells. When they are tense their walls do not 

 act as mere props or stays, as do the floors or partitions of a house ; 

 but the turgor gives individual rigidity to each cell, and through 

 them collectively to the whole part which they construct. A high 

 mechanical effect is thus gained in a succulent structure with extreme 

 economy of material. This may be illustrated by the case of the 

 Lettuce. We have seen that a crisp Lettuce suitable for a salad gives 

 by weight 96 per cent, of water, and only four per cent, of organic 

 material for cell-walls and protoplasts. Such a structure is in fact a 



