2 ON THE MECHANISM OF LEAVES. 



larirE, and also many of the twining plants ; I shall then 

 show the manner in which most leaves that are nearly sessile 

 are conducted, and what sort o( motion they possess, 

 giving as an example the ericae ; and shall finibh by the 

 evidence of a few more leaves, to prove, that there are hardly 

 any vvithout some sort of mechanism. 

 Mechanical "To begin with the species of leaves, which embrace tlie 



arrangemeiit stem, 1 shall produce as an example one of the umbelli- 

 einbracin<^ the ^^rous tribe ; the smyrnium blusatrum. This plant has at 

 sietn. the end of the leaf a large hood, whiclv X ^WU call the pro- 



tector; because it not only coiitraicts 'and dilates like a 

 gatherer (the spiral wire running-^in stripes through it) but 

 serves as a guard to the buds ; which, forming in bunches, 

 stay not within the bark, as the buds of trees, till fit to issue 

 forth, but shoot, like all annuals and herbaceous plants, 

 direct-ly from the line of life to the exterior of the rind. 

 Nature seems therefore to have placed this sort of cover 

 (see PI. 1, fig. 1, <ia) which grows with its growth, and 

 clings close to it, as a succedaneuro for the stem, which in 

 trees covers and conceals the bud for a long time. And it 

 can scarcely be conceived what a perfect protection it is 

 from the frosts of spring, as, like all leaves, it has on each 

 side that impervious skin or cuticle, which no rain or cold 

 winds can pierce, ^ud no moisture pervades, but that which' 

 - passes through the hairs into the leaf, and that which evapo- 

 ration gives it. Most of the pentandria digynia tribe are 

 formed thus, at least with leaves possessing this species of 

 mechanism. It may easily be seen how much the spiral 

 wire is contracted and turns, sifice, whether opeit or shutt 

 the protector is continually twisted in a double manner 

 round the stem ; nor does the large bunch of flowers leave 

 its close drawn curtain, till just before the corollas open. 

 Being umbelliferous, large branches of flowers shoot at 

 once ; as soon as the cover is withdrawn, new life as well as 

 light seems to be given to the plant within, which is, when 

 first opened, generally found covered with a white powder, 

 probably the result of evaporation. Of this I shall hereafter 



Peiitandria ^\ve a farther account. But when the leaflets in this plant 

 ikgy ma plants. ^ ,■,-,, 



increase to a great length (which they will often do to ten 



or twelve leaves of a side) though they in some measure 



resemble 



