554 SECONDARy CHANGES. 



action of the two processes the whole cortex lying outside the pericambium or 

 endodermis is as a rule split, and thrown off, suffering immediate decay in the case 

 of subterranean roots, while it adheres to parts above the ground in the form of flaps, 

 which gradually dry up. In parts which swell quickly this is very conspicuously ap- 

 parent ; the separation of the flaps from one another often takes place exactly along 

 those longitudinal lines iri which the cambiogenetic growth in thickness is at first most 

 vigorous, and consequently in those lying opposite the primary phloem-bands of the 

 vascular bundle. As in diarch main roots the latter alternate with the cotyledons, in 

 such cases a separation into two flaps occurs, each of which lies below a cotyledon, 

 a phenomenon which has often been described in the case of plants forming tuberous 

 roots which, together with the hypocotyledonary stem, swell rapidly; it was com- 

 pletely explained by Turpin in 1830 ^ Where the swelling is less rapid, the process 

 under consideration, by which the usually relatively voluminous primary cortex is 

 thrown off, often results in a diminution of the total thickness of the member, which 

 is only compensated by later secondary growth ; most roots of Dicotyledons and 

 Gymnosperms are thinner at the beginning of the secondary growth than before, 

 owing to the loss of the external cortex. 



The commencement and progress of the cell-divisions in the periderm of roots, 

 though it has not been investigated in all its details, yet corresponds, so far as is 

 known, to the general rules for the formation of periderm. According to the data 

 made known by Van Tieghem, their products are in all cases, including the Coniferas, 

 both phelloderm and layers of cork. The former constitutes an external layer of the 

 parenchyma, and is always of relatively small bulk. The cork-cells generally form 

 thin integuments, rarely (e.g. Pistacia Lentiscus) more bulky, cracked masses of 

 cork. In the more or less fleshy roots of herbaceous plants, the continuous 

 suberous integument is often extraordinarily thin, being reduced to one or two 

 layers of cells. The peeling off and decay of the outermost layers, for the time 

 being, are no doubt considerably accelerated in the case of subterranean roots 

 by the nature of their surroundings. 



In the roots of woody plants the general character of the suberous integument 

 is similar to that of the stem. No minute investigations exist on the special differences 

 which may occur between the two in the same plant. In many herbaceous plants 

 the relatively great irregularity in the arrangement of the whole peridermal layer is 

 conspicuous. , 



The cells of the endodermis, which are likewise wholly or partially suberised, 

 often forrn the outermost stratum of the cork-layer, when the external cortex begins 

 to be thrown off, and, on further growth, are the first to be thrown off themselves. 



Sect. 177. Repeated formation of internal periderms. The first formation of 

 periderm, whether it be internal or superficial, is in many woody plants the only one, 

 and the periderm follows the growth in thickness of the parts enclosed by it : Fagus. 

 The great majority of ligneous plants, however, form on stem and branches new 

 internal periderms, after the first one, which arise successively in deeper layers of the 

 cortex, and cut off successively deeper zones of tissue as dry bark. This process 



' Turpin,. Ann. Sci. Nat. i sen torn. XXI. p. 298, pi. 5. Compare Botan. Zeitg. 1873, pp. 

 129, 297. 



