212 R. BRAITHWAITE ON THE HISTOLOGY OF PLANTS. 



in the third, we get the tabular cells, roundish, polygonal, or 

 stellate, so well seen in the cuticle of leaves, and in the partitions 

 of the air passages in rushes or in Alisma Plantago ; in cork -tissue 

 they are nearly rectangular, hexagonal in the leaves of Hepaticae, 

 and beautifully radiate in many Desmids, such as Euastrum 

 MicrasteriaSy &c. If growth goes on in only one of the cell 

 diameters, we get long cylinders, as in the whole group of filiform 

 Algas, in the pallisade parenchyma of leaves, and vessels of higher 

 plants ; or prisms in vessels and wood and bast parenchyma ; or 

 filiform in many hairs, of which cotton is a familiar example. 

 Irregular extension produces branching of the elongate cells, as 

 seen in the bast cells of Asclepiads and in the Cinenchyma or 

 laticiferous vessels carrying the milky sap of composite and 

 papaveraceous plants, which ramify and anastomose repeatedly. 



Following immediately on extension of the cell is a thickening 

 of the cellulose case, which may take place equally on all sides, 

 as in pith and wood cells, or may be more or less confined to 

 one side of the cell. The amount of deposit may be so slight as 

 to be scarcely noticeable, or in the case of wood and bast cells, it 

 may fill up and almost obliterate the entire lumen or central cavity 

 of the cell. The thickening of the cellulose case on one side is 

 best seen in cells of the epidermis, where it may occupy the whole 

 of the wall on one side, or also parts of the two adjacent walls, as 

 is seen in the. cuticle of Mistletoe, or of Allium, Hyacinth, &c. 

 A partial thickening in the angles is seen in Hepaticee, and in the 

 collenchyma of young cabbage stems. 



Lamination of the Cellulose. — As a rule, the different parts of 

 the cellulose case, originating at different periods, and increased 

 by deposit of cellulose particles separated from the protoplasm, and 

 placed uniformly over the primordial membrane, differ also in their 

 optical character, and for this reason we may distinguish three 

 coats. 1. The primary, which is thickest; 2. The secondary 

 deposit in laminae, or systems of laminae ; and 3. The tertiary, 

 youngest and most internal. Sulphuric acid or caustic soda will 

 bring out the laminte of the secondary deposit when not evident, 

 and an interesting question arises, How is this lamellation caused ? 

 Nageli says in the same way as in starch granules ; by the 

 differentiation of an originally homogeneous deposit into layers 

 containing different amounts of water, but the older view is, that 

 in the process of deposit certain periodical stops take place, each 



