192 TRANSACTIONS, NATJIAL HISTORY SOCIETY OF GLASGOW. 



it brings the centre of gravity nearer the point of 

 support. One striking example may be quoted from 

 his article in support of the views advanced in this 

 paper regarding the use of the subdivision of leaves. 

 The mountain-ash has compound pinnate leaves, 

 while the other members of the genus Pyrus, to 

 which it belongs, have simple undivided leaves. Sir 

 John can only account for this by the fact that the 

 mountain-ash grows on bleak exposed situations. 

 The unsteadiness of the aspen leaf is i)roverbial, but 

 is easily explained by its thin laterally-compressed 

 petiole. Lyrate leaves, and those having large ter- 

 minal lobes, like Heracleum, are set in motion by 

 the slightest breeze. The fact has sometimes been 

 remarked that more trees are overturned by a violent 

 storm occurring before the leaves have fallen, than 

 later in the year when the trees have become bare 

 and leafless. 



A brief reference to the morphology of steins 

 must suffice for the present. Various terms are em- 

 ployed to describe the transverse sections of stems, 

 e.g., terete, coinpressed, square, triangular, fluted, 

 &c. In order x^^'operly to compare the powers of 

 resistance to lateral strain of these various forms, 

 we should require to leave out of account differences 

 due to consistency of material, and consider only its 

 amount and distribution, although it is certain that 

 the resistance of stems to a breaking or bending 

 force deiDends mainly on the consistency of the 

 materials composing them. As we have already seen, 

 the strength of a column is proportionate to the 

 area of its transverse section. The round or terete 

 stem, which exposes the least surface to the wind 

 in proportion to its thickness, is to be preferred. 

 The compressed stem is the weakest, and is weak in 

 proportion to the difference between its two 

 diameters. Com^Dressed stems are not common. 

 Tlie scape of the daffodil may be taken as an 

 example. This plant usually grows in situations 

 somewhat exposed to the wind. Few flowers have a 



