SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. ^^'^ 



maining cases it has not been found, or is not mentioned. The cork-cells form 

 collectively thin integuments, consisting of a few layers. The extremely irregular 

 arrangement, still requiring closer investigation, of the thin suberous integument of 

 Cobaea is worthy of remark. 



As regards the very various details of the first development of the periderm, and the 

 structure of its mature parts, we must refer to the monographs. Here some special 

 cases only may further be described as examples. 



The above-mentioned Cupressinex, and no doubt their similarly constructed allies also, 

 have in the young internodes more or less broad, blunt projections of the external cortex, 

 running down from the leaves, and traversed by vascular bundles or resin-passages 

 (Juniperus), or both, which pass obhquely to the next leaf above. The outline of the 

 bast, as seen in transverse section, is circular. The initial layer of the periderm passes 

 close by the bast, — its position still requires more precise determination, — and is only a 

 few layers of cells distant from the epidermis between the foliar projections, but is far 

 removed from it opposite them ; the whole of the projections, with their bundles and 

 resin passages, are cut off by the layer of cork. 



The Abietinece, so far as is known, behave very variously as regards the first formation 

 of periderm. Abies pectinata, as stated above, forms its periderm in the sub-epidermal 

 layer of parenchyma. According to Mohl's statements ^, which however do not enter 

 very minutely into this point, the same conditions may be assumed to exist in A. 

 sibirica and Pinus Strobus. A contrast to these is presented by Larix europaea, where 

 the parenchymatous foliar pulvini, which contain resin-canals, are thrown off by internal 

 formation of periderm in the first year. Whether the periderm is also deeply-seated 

 between the pulvini, or whether, as conjectured by Sanio, it is here immediately hypo- 

 dermal, as in the furrows of Casuarina, is not known. Abies excelsa, Pinus silvestris and 

 nigricans behave, according to Mohl, in a similar way to the Larch, as regards the first 

 formation of periderm in the pulvini, with the sole difference that the periderm, especially 

 in Pinus, is inserted less deeply and lies outside the resin-canals. 



Casuarina ^ has large longitudinal projections, separated by narrow furrows, on the 

 internodes of the branches. The approximately cylindrical bast is surrounded by a 

 broad zone of parenchyma containing little chlorophyll, which in the furrows reaches to 

 the epidermis, but in the projections is separated from the latter by the masses of tissue 

 described already, through which runs the vascular bundle of the next segment of the leaf 

 above, almost at the level of the base of the furrow. 



The formation of periderm begins in the furrows, and in fact in the sub-epidermal 

 layer of parenchyma. It is continued from each furrow in both directions, through a 

 layer of the internal parenchyma directed towards the vascular bundle, and finally 

 through a band of cells running transversely through the phloem of the bundle. Thus 

 in the furrows the epidermis only is cut off by the peridermal layer, while between them 

 this is the case with the whole of the foliar projections, including the external part of 

 their vascular bundles. 



2. Internal primary formation of periderm is a quite general rule in those rools 

 which grow in thickness according to the Dicotyledonous type ^, and its initial layer 

 is here always the pericambial or rhizogenetic layer of cells, which adjoins the 

 endodermis on the inside. In the seedling it extends in many cases upwards 

 from the root, through the hypocotyledonary stem. Its beginning coincides with 

 that of the more abundant cambial growth in thickness, and through the joint 



^ Botan. Zeitg. 1859, p. 337. ^ Sanio, I.e. — Low, I.e.; compare p. 256, 



^ Van Tieghem, Symmetric de Struct., I.e. 



