88 STRUCTURAL BOTANY 



mechanical tissue offers the greater resistance to bend- 

 ing strain, the further it is removed from the centre- 

 line, or " neutral axis," as it is called in mechanics, of 

 the column, here represented by the stem. For this 

 reason we know that iron columns are always made 

 hollow, for the same amount of material can be used to 

 better advantage if brought as near the exterior as 

 possible, than if distributed all over the transverse 

 section. This mechanical principle is constantly illus- 

 trated in the construction of plants. Again, the 

 assimilating tissue obviously requires to be as near the 

 surface as possible, so as to be fully exposed to light, 

 without which its work cannot go on at all. 



Now we will see how in the stem or in a branch of 

 Equisetum a compromise is made between these two 

 competing interests. Each prominent ridge of the stem 

 is occupied by a strand of sclerenchyrna, and there are 

 an equal number of additional strands placed at the 

 bottom of the furrows (see Fig. 39, sc). The assimilat- 

 ing tissue occurs in curved bands, each of which lies 

 behind one of the sclerenchymatous ridges, and reaches 

 the surface on either side of it, between the mechanical 

 tissue of the ridge and that of the furrow (see Fig. 39, a). 

 The epidermis has stomata at those places only where the 

 chlorophyll-tissue reaches the surface, so they are placed 

 where they are most needed for transpiration and the 

 passage of gases. We notice also that the bands of 

 chlorophyll - tissue lie directly opposite the vascular 

 bundles, so that they are well situated both for receiving 

 the water and mineral substances from the' latter, and 

 also for transferring to them in return the products of 

 assimilation. In the very minute ultimate branches, 

 such as that of which a transverse section is shown in 



