PRINCIPLES OF VEGETATION. 343 



that purpofe. From this period the plant is to expe<5l a fupply from the 

 root, and to derive nourifhment by means of its cxtenfive ramificationSj 

 and their abforbent properdes. 



We proceed now to neticc the ftrudlure of plants: and firft of the root. 



In examining the Root of plants, the firft thing that prefents itfelf is 

 the fkin, which ir. of various colours in different plants. Every root, arter 

 it has arrived at a certain age, has a double fkin. The firft is coeval 

 with the other parts, and exifts in the iced ; afterwards a ring fent off 

 from the bark forms a fecond fkin ; e.g. in the r^ot of the dandelion, to- 

 wards the latter end of May, the original or outer fkin appears fhri- 

 velled, and is eafily feparated from the new one, which is frefher, and ad- 

 heres more firmly to the bark. Perennial plants are fupplied with a new 

 (kin every year; the outer one falls off in the autumn or winter, and a new 

 one is formed from the bark in the fucceedins; foring. The fkin hasnu- 

 merous cells or veffels, and is a continuation of the parenchymatous part 

 of the radicle. However, it does not confift folely of parenchyma, for 

 the microfcope fhews that there are many tubular ligneous veffels inter- 

 fperfed through it. • 



When the fkin is removed, the true cortical fubftance or bark appears, 

 which is alfo a continuation of the parenchymatous part of the radicle, 

 but greatly augmented. The Bark is of very different fizes. In moft 

 trees it is exceedingly thin in proportion to the wood and pith. ' On the 

 other hand, in carrots, it is almoft one half of the femidiameter of the 

 root J and in dandelion it is nearly twice as thick as the woody part. 



The Bark is compofed of two fubftances, the parenchyma, or 

 pulp, which is the principal part, and a few woody fibres. The paren- 

 chyma is exceedingly porous, and has great refemblance to afpunge; 

 for it fhrivels confiderably when dried, and dilates to its former dimen- 

 fions when infufed in water. Thefe pores or veffels are not pervious fo as 

 tp communicate with each other, but confift of diftinct little cells or blad- 

 ders, fcarcely vifible without the affiftance of the m.icrofcope. In all 

 roots, thefe cells are conftantly filled with a thin watery liquor. They are 

 generally of a fpherical figure, though in fome roots, as the buglofs and 

 dandelion, they are oblong. In many roots, as the horfe-radifh, peony, 

 afparagus, potato, &c. the parenchyma is of one uniformi ftrudburej but 

 in others it is more diverfified, and puts on the fhape of rays running 

 from the centre towards the circumference of the bark. Thefe ravs 

 fometimes run quite through the bark, as in lovage; and fometiiiies ad- 

 vance toward the middle cf ii, as in melilot and moft of the leguminous 



3M 2 and 



