MECHANICAL CONSTRUCTION OF PLANT-BODY 173 



large types some accessory means of strengthening has to be adopted, such 

 as matting the branches together as in Codium and Penicillus : or cementing 

 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 (Fig. 32, p. 54) ; and in the young 

 embryo-sacs of the Flowering Plants. (See p. 295.) 



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 



Fig. 107. 



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

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

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



the cells are seen to divide into equal parts, each newer cell-wall 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. 107). The disposition of the walls is from the first such 

 as to give added mechanical strength, together with other advantages. 

 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 



