HAY-TIME 155 



where the wind would act most violently. The centre 

 of gravity is brought nearer to the ground. The 

 haulm is stiffest where the overturning moment is 

 greatest, most flexible where the spikelets are situ- 

 ated. It is important to many grasses for the dis- 

 persal of their pollen and seeds that the spikelets 

 should dance in the wind. 



The diaphragms of the haulm have, as we noticed, 

 some effect in stiffening the structure. When a 

 hollow cylinder is bent, the opposite surfaces tend to 

 approach the neutral line, and the cross-section 

 becomes elliptical. Whatever resists that change of 

 shape, such as a solid floor, will oppose bending. 

 The diaphragms are most crowded where excessive 

 bending would be most injurious, i.e., near the ground, 

 It is probable that the diaphragms offer no appre- 

 ciable resistance to moderate bending. 



The mechanics of a long bone cannot be treated with- 

 out raising some of the same questions. This subject 

 has already been handled by Dr. Donald Macalister 

 in a lecture which is peculiarly interesting and at the 

 same time perfectly simple. I should merely refer 

 the reader to his article in the English Illustrated 

 Magazine^ if it were not that every reader has not 

 ready access to the old volumes of a periodical. 



Dr. Macalister points out that the tubular form of 

 a long bone, such as the human thigh-bone, fits it to 

 resist either a breaking or a crushing stress. A solid 

 cylinder of the same mass would be weaker than the 

 hollow cylinder. If the solid cylinder had a diameter 

 of 100 units, and the hollow cylinder an external 

 1 1883-4, p. 640. 



