l66 LECTURE X. 



grow first in the radial direction, and then become divided by tangential longitudinal 

 walls. Of the two daughter-cells so produced, the outer developes into a cork-cell ; 

 while the inner one, continuously increasing in width radially, again divides. Of 

 the two daughter-cells now existing, the outer one is again transformed into a 

 cork-cell ; while the inner one remains active, and repeats the same process. 

 This layer of cells which continually become regenerated is the phellogen; the 

 similarity of which to the cambium is increased yet more by the fact that layers 

 of tissue often originate from the phellogen on the inner side also. These, as 

 the so-called PheUoder7n, strengthen the parenchymatous living cortex (cp. Fig. 171). 

 The arrangement in radial rows and at the same time in concentric layers (which 

 immediately results in part from the origin of the cork-cells mentioned above, and 

 in part from their quickly terminated growth) are almost universally characteristic of 

 cork-tissue. 



As in many cases {Viscian, Acer sln'afwn, species of Coniiis, SiC.) the epidermis 

 itself follows the growth in thickness of the organ for many years; so also the 

 periderm (and this is oftener the case) may for years follow the increase in 

 circumference of a branch or stem by passive stretching and growth. 



Sooner or later^ however, Avhere the growth in thickness persists for a long 

 time, a repeated changing of the peripheral tissues usually results — Bark is formed. 

 In rarer cases this occurs as so-called ring-bark. This arises from the development 

 of new layers of periderm in the deeper cortex, some cell-layers distant from the 

 preceding periderm ; and the older layers thus become exfoliated across the stem 

 ( Vi'/i's, Clemalis, Cupressinese, according to De Bary). Scale-bark, of which we find 

 the simplest and most instructive example on the stems of Plalanus, is however 

 commoner. As is well known, there peel off from these stems in the summer 

 larger or smaller plates of tissue, often as large as the hand and 1-2 mm. thick; 

 and a somewhat rough surface, beset with corky warts, is left behind on the^tem 

 after their fall. If we now suppose that, at the same places in the cortex, such scales 

 gradually die, and do not fall off, but remain suspended and clinging to one another 

 on the stem, the latter would, after some years, become surrounded with a scaly 

 layer of dead masses of tissue, deposited one over the other. These together constitute 

 scale-bark; and at the same time, in consequence of the progressive increase in 

 circumference of the stem, these dead packets of scales become more and more 

 separated from one another by means of longitudinal clefts running between them. 

 This process is to be recognised very clearly on close observation of the bark of old 

 Pine stems. The origin of these scales of bark is brought about by the repeated 

 formation of thin lamellcC of cork within the living cortex ; these do not however run 

 all round the organ, l)ut their edges come to the surface at circumscribed places, so 

 that a portion is cut out of the cortex, so to speak, as if with a gouge. Not only the 

 phellogen-layer concerned, but also the whole of the cortical tissue lying on its 

 exterior dies off and dries up; whereupon the same process is again repeated towards 

 the interior. Since now these cork-lamellas cut out indiscriminately the most 

 various forms of tissue from connection with the living parts, the bark-scales con- 

 sist of all the different cell formations which occur in the cortex generally ; and it 

 is clear that, since by the progressive development of bark a thicker and thicker coat 

 grows around the stem, the living cortex itself, whilst being regenerated in its turn 



