xxxvti ACTIVITIES AT 70 YEARS 203 



the strain is equal. If not, it is obvious that a certain 

 amount of material may be removed from the strongest 

 part without increasing the danger of rupture. If the 

 stem of a plant, or any other pillar, is affected by pres- 

 sure — say of wind — one side will be extended and the 

 other compressed, while between them will be a neutral 

 axis, and both extension and compression will be greatest 

 along the surface farthest from the neutral axis. It 

 follows, therefore, that the strongest form is where the 

 material is collected as far as possible from the neutral 

 axis. The two bars cannot, however, be entirely 

 separate, and must, therefore, be connected by a bar 

 or bars. This is the origin of the well-known girder. 



If the forces to be resisted act in two directions at 

 right angles to one another, two girders must be com- 

 bined, one at right angles to the other. 



If the forces act in all directions, a circular series of 

 girders will be required, as Schwendener and others have 

 pointed out. This is the case in the stems of trees, 

 where the woody fibres form a ring, only separated in 

 places by what are known as the " medullary " rays. 

 This is the reason for the prevalent round form of stems. 



The question then arises, Why is this form not 

 universal ? As regards plants having quadrangular 

 stems, it may be pointed out that when the leaves were 

 in opposite pairs, each pair at right angles to those 

 above and below, as, for instance, in the dead nettle, 

 the strain of the wind would be mainly in two direc- 

 tions, and the "double girder" would be the best 

 form. If so, we should expect to find quadrangular 

 stems associated with opposite leaves. The author 

 then took the British flora, and showed that plants 

 with quadrangular stems always have opposite leaves, 

 and that plants with opposite leaves have generally, 

 though with exceptions, quadrangular stems. The 

 reasons for these exceptions were then considered. 



Passing to triangular stems, it was pointed out that 

 they might be accounted for by the same considerations. 

 Many Monocotyledons, but not all, have the leaves in 

 threes. Sedges, for instance, all have more or less 

 triangular stems, while in grasses they are round. 

 Now, sedges have leaves in threes, while in grasses they 

 are in two rows or ranks. 



In plants with pentagonal stems the same relation 



