182 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



Bridge, engineers have been compelled to take 

 account of wind-pressure in their calculations. The 

 subdivision of leaves may in many cases have been 

 determined in relation to the same end. 



6. Branched Forms. — Leaves belong to the discoid 

 type since their subdivision takes jDlace in one plane. 

 The branching of a stem, on the other hand, occurs 

 in several different planes. In the majority of trees 

 the general contour or outline is spherical. The 

 spherical contour is what we should naturally ex- 

 pect as the development of a globular oospore. The 

 subdivision of the sphere is necessitated by the 

 plant's dei^endence on air and light ; but the branches 

 going off alternately, in different planes and at 

 different levels, offer no great resistance to the 

 wiiid, which readily finds a passage through the 

 tree. In very tall trees with columnar stems we 

 have a departure from the spherical contour calcu- 

 lated to endanger stability. The elongated trunk 

 simply gives the wind a more jDowerful lever for its 

 overthrow. Especially is this true when there is 

 a large expanse of leaf-surface at the apex of a 

 lofty stem. Hence the cylindrical form is more 

 frequent among herbaceous plants of no great size, 

 growing together in masses, than among lofty trees. 

 Flat-topi^ed herbs and shrubs are common, but flat- 

 topped trees are rare. The palm stem has evidently 

 acquired exceptional tenacity and elasticity to meet 

 this emergency. The case of the pine is perhaps 

 more difficult to explain, especially when we consider 

 the immense height attained by some species, and 

 the exj)osed situations where they gro^v. It is 

 impossible, however, to regard a pine-tree attentively 

 without being struck with the disproportion in 

 thickness between the stem and the branches. You 

 find no very thick boughs on a pine-tree, such iis 

 abound on the oak, beech, and elm. Moreover, 

 in the j^ine tree the lower branches are very 

 much larger than the upper, and the tree tapers 

 rapidly. Like close-clewed topsails, the needle-shaped 



