36 THE PLANT CELL. 



B. CELLS ARISING FROM THE PERIBLEM. 



1. The Cortex. 



The cortex will be de.scribed here under the external tissues, 

 but, strictly speaking, it is only an external tissue when con- 

 sidered relatively to the central cylinder (where the latter is 

 present) ; nevertheless, it is convenient to class it with the 

 external tissues. Cortical cells are formed by the subsequent 

 growth and modification of cells of the periblem — viz., that 

 layer which is just internal to the epidermis in the young shoot. 

 Each adult cell of the cortex is, in most cases, a typical assimilat- 

 ing cell, Avith the exception that cortical cells in roots do not 

 contain chloroplasts, but plastids. As a rule, the shape of a 

 cortical cell is oval, or more often rectangular or polyhedral in 

 section, and such a cell would be termed parenchymatous, since 

 no diameter is much in excess of the others. 



In stems which possess a well-marked central cylinder the 

 cortex extends radially from the epidermis (or hj'podermis) to 

 the endodermis or starch-sheath. In herbaceous stems all the 

 cells may possess chloroplasts, but where the layer is of any 

 extent only the outermost cells possess chlorophyll. At times 

 a well-defined layer of cells possessing chlorophyll is met with 

 at the outer margin of the cortex, this being known as the 

 phelloderm ; but it is formed from a tissue known as cork- 

 cambium, and, as such, will be examined later. 



2. Cells of the Mesophyll of Leaves. 



The mesophyll in leaves is that tissue which exists between 

 the epidermis of the upper and under surfaces, in the case of the 

 bifacial leaf, and in the centric type is the mass of cells which 

 intervenes between the epidermis and the central cylinder. 

 A layer of columnar cells known as palisade parenchyma is often 

 present between the true mesophyll or spongy parenchyma and 

 the epidermis of the upper surface of a bifacial leaf, and these 

 palisade cells are characterised by the presence of large numbers 

 of chloroplasts (see Fig. 1 5a). Each cell of the spongy paren- 

 chyma is thin-walled and possesse.s protoplasm and chloroplasts, 

 and is chiefly concerned in the processes of transpiration, and 



