R. BRAITHVVAITE ON THE HISTOLOGY OF PLANTS. 279 



2. — Pith Tissue may be considered as the first advance from 

 the primary parenchyma, from which the cells differ but little in 

 form, but become somewhat elongated, and acquire a secondary 

 thickening of the cellulose case. The pith occupies the centre of 

 the axial organs in the higher plants, and in monocotyledons and 

 vascular cryptogams forms the intermediate tissue which separates 

 the inner vascular bundles from each other. 



Pitli cells vary greatly in form, and thus various names have 

 been given to the various kinds of pith tissue, as incomplete paren- 

 chyma, when the cells touch each otber only at separate points, 

 and according to the form of cell we have rounded parenchyma, 

 as in the pith of peony, spongy in that of canna, and the middle 

 of leaves, and stellate in the rush. In complete parenchyma the 

 cells are closely in contact, so that intercellular spaces are absent, 

 or if present narrow and triangular ; one variety is the regular 

 parenchyma, where the three diameters are nearly equal, and this 

 by the shortening of one of them becomes the muriform tissue, 

 while elongated parenchyma is more particularly found in the pith 

 of such quick growing plants as the kidney bean, as well as in 

 that of conifers. 



Slight thickening is seen on the cellulose wall of many pith cells, 

 as in the elder, and less frequently it is more developed, as in 

 Fraxinus, Cyccis, &c., while in a few cases, as in Clematis and 

 Hoya there is great thickening. 



The air passages arise by resorption of rows of cells often 

 abnormally formed, and are well seen in water-lilies and other 

 aquatic plants, and occasionally a few scattered bast bundles and 

 milk vessels may be found in pith. 



The pith continues active during the whole life of the plant, and 

 constitutes a store of reserve material during the resting period 

 for the changes set up at the time of renewed growth. 



3. — Cortical Tissue. — This includes that portion of the stem 

 lying between the fibro-vascular bundles and the epidermis or cork, 

 and in leaves between the cuticle and vascular bundles of the 

 nerves ; it is therefore most distinct in parts exposed to the air 

 and light The primary bark proceeds directly from the primordial 

 tissue of the growing point, and rapidly increases by cell division. 

 In annual plants it is completed simply by extension of these cells, 

 as it is also in perennial plants which cast off their bark by cork 

 tissue arising under it; but in those like the holly and the mistletoe. 



