I INTRODUCTION 37 



In most cases the stems of plants are round, but in 

 others they are triangular, quadrangular, pentagonal, 

 elliptical, or of other forms. The bare fact is stated in 

 many books of botany, but I have not come across any 

 in which an explanation is given, or even attempted. 

 Yet it will, I suppose, be generally admitted that there 

 must be a reason for all these differences. 



If an engineer is constructing a bridge, or an 

 architect is building a house, he must consider how to 

 make it safe, and secondly, how to economise the 

 material as much as possible. -The strongest form 

 which can be given to a solid body, in the formation 

 of which a given quantity of material is to be used, 

 and to which the strain is to be applied under given 

 circumstances, is that form which renders it equally 

 liable to rupture at every point. So that when, by 

 increasing the strain to its utmost limit, the solid is 

 nearly breaking at any one point, it may be nearly 

 breaking at every other part. If there are any parts 

 which are not nearly at the eve of rupture it is 

 obvious that from such points some of the material 

 might be removed without increasing the danger of 

 rupture. 



If the stem of a plant is acted on by wind, one side 

 will be extended, the opposite compressed, while between 

 them there will be a neutral surface ; and both extension 

 and compression will be greatest along the surface 

 furthest from the neutral axis. It follows, therefore, that 

 the material cannot be in the state bordering on rupture 

 at every point of a section at the same time, unless all 

 the material of the compressed side be collected at the 

 same distance from the neutral axis, and likewise all 

 the material of the extended side, i.e. unless, in the 

 words of Professor Moseley,'- " the material of the ex- 

 tended side and the material of the compressed side 

 respectively be collected into two geometrical lines 

 parallel to the neutral axis." The two bars are in 

 practice connected by a cross rod, or web as it is techni- 



' See Moseley, Engineering and Architecture. 



