36.] CHAPTER II. THE TISSUES. 213 



brownish or whitish specks. When the periderm of the stem is 

 superficial, the lenticels are developed under the places where 

 the stomata occur in the epidermis, and these spots are commonly 

 the starting-points of the formation of the periderm ; but this is 

 not the case in stems with a deep periderm, nor is it ever the 

 case in roots. In many trees, as the Birch, the lenticels become 

 much extended in width by the growth of the branch in circum- 

 ference. When the periderm is very thick, as in the Cork-Oak, 

 the lenticels form deep canals filled with a pulverulent mass of 

 cells. Sometimes lenticels are not formed ; they are not present 

 in the stems of some plants which have a pericyclic phellogen 

 (e.g. Vitis, Clematis, Rubus, Lonicera). 



The phelloderm or secondary cortical tissue, the tissue formed 

 internally from the phellogen, consists of cells which have 

 essentially the same structure as those of the primary cortex : the 

 secondary cortex can, however, be distinguished from the primary 

 by the regular radial rows in which, like those of the periderm, 

 its cells are arranged. The cells have protoplasmic cell-contents, 

 and, when developed near the surface of aerial stems, they con- 

 tain chloroplastids : their walls are usually thin and consist of 

 cellulose, but, like those of the cells of the primary cortex, they 

 may become more or less thickened and eventually lignified. 



Just as the periderm replaces the disorganised epidermis as a 

 tegumentary tissue, so the phelloderm replaces the primary 

 cortex as a nutritive (metabolic) tissue when the primary cortex 

 becomes obliterated under the conditions explained on p. 207. 



It may be noted that, in the stem of Isoetes, the secondary cortex is developed 

 on the outside of a layer of meristem which, at the same time, forms vascular 

 tissue internally (p. 205) : the amount of the former very greatly exceeds that 

 of the latter. 



36. Formation of Tissue in consequence of Injury. 



When the internal tissues of most parts of plants are laid bare by 

 injury, they are gradually covered by a formation of cork taking 

 place in the outermost layer of cells which remain uninjured and 

 capable of growth. This is easily seen in injured fruits, leaves, and 

 herbaceous stems, in which the wounds that have been covered by 

 a layer of cork are distinguished by a grey-brown colour. The 

 process is very easy to observe in potato-tubers, for each portion 

 of living tissue taken from one, if only prevented from drying too 

 quickly, will soon be covered over the whole surface by a layer of 



