220 MORPHOLOGY. 



drocharis Morsus ranee, according to Meyen. Simultaneously with the 

 origin of the root under the bark (fig. 155. e\ a cellular layer (fig. 156. 

 b\ completely enveloping the little cone constituting the root, quite 

 down to its base, separates wholly from the cortical parenchyma in these 

 plants, still, however, retaining its vitality, and in vital connection with 

 the extreme point of the root ; the cellular tissue of the apex of the root, 

 and that of the cap of the root, always passing into one another here. 

 Under natural conditions, this cap of the root is persistent during the 

 whole life of the root, but if torn off it is never reproduced, and the 

 root dies. 



In some parasites, e. g , in Cuscuta, and also frequently in ffedera, 

 the bark swells out into a disc (sucker, haustoriuni) over the developing 

 adventitious root, and this, originally applied flat upon the foreign body, 

 subsequently becomes concave, from the especial extension of its bor- 

 der, and (exactly as in the sucking-disc of the leech, or the pro-legs of 

 caterpillars) attaches the parasite to the support by a vacuum. From 

 the bottom of this disc springs the adventitious root, which, if it pro- 

 ceeds forward, penetrates the supporting body. 



Comprehensive comparative researches into the anatomical structure 

 of adventitious roots are still a desideratum. The only accurate ones we 

 at present possess are from Mohl * and Mirbel t on the Palms. 



125. The varieties of form in true and adventitious roots are 

 not very manifold, and they depend on their direction, arrangement, 

 as well in regard to the stem as among themselves, preponderating 

 formation of parenchyma in particular places, and formation of 

 wood, through the indefinite vascular bundles of the Dicotyledons. 

 No root is capable of producing buds. In a large portion of the 

 Monocotyledons, especially in the Grasses, and all those in which 

 the seed is furnished with an operculum (see, hereafter, under the 

 head of the Seed), even in some Dicotyledons, for instance, Nelum- 

 bium, the radicle is not all developed in germination. Conse- 

 quently, these have no true root ; in place of this, adventitious roots 

 are immediately formed (see the preceding paragraph). 



All botanists fully agree, that everything developed from the plumule 

 and buds, above the cotyledons (leaves, and the readily distinguishable 

 aerial roots, as they are called, exceptedj, is to be reckoned as part of 

 the ascending axis ; but at one time they counted among roots, the bulb, 

 tuber, rhizome, many- headed root, premorse root, &c., clearly parts which 

 are developed from buds above the cotyledons; or raised an endless con- 

 tention, on manifestly unsustainable ground, as to whether these parts 

 are roots or not : certainly a right substantial proof of what perversities 

 are induced by the neglect of correct method, and the one-sided con- 

 templation of a solitary stage of formation torn from its normal con- 

 nexions. Most of these forms are now correctly disposed of, but a few 

 botanists still hold to a part of the old beaten path. J 



* De Structura Palmarum. Munich, 1831. 



f Nouvelles Notes sur le Cambium, Paris, 1839. 



j Link (Philos. JBotan. edit. 2. vol. i. p. 361.), for instance, still retains the radix 



magnified, a, The root, in which may be distinguished a central vascular bundle, and 

 a thicker bark, b, Layer connected with the apex of the root by persistent cellular 

 tissue, free in the remaining portions. 



