R. BRAITHWAITE ON THE HISTOLOGY OF PLANTS. 215 



little pill's heads into the cell lumen ; these are produced like the 

 [lores of wood cells by a fold of the young cellulose case. In 

 petals, and especially well seen in the papilliforni epidermal cells 

 on those of the Pelargonium, we have both folding of the primary 

 cellulose case, and also secondary deposit. In the epidermis cells 

 of the leaves of many Urticacea?, the thickening is limited to one 

 point, but this grows remarkably, forming spheroidal cellulose 

 bodies, termed cystoliths, which are encrusted with calcareous 

 crystals ; good examples of these will be found in a leaf of Ficus 

 elastica, or Indiarubber plant, where they are seen on section to 

 occupy a large crypt, from the roof of which they hang by a 

 slender pedicel, and a distinct nucleus is visible in their interior ; 

 the leaves of Broussonetia and Morus also afford specimens. 



Origin of the various Forms of Thickening. — You will remember 

 when speaking of the young cell, that I alluded to the currents of 

 protoplasm traversing the cell walls in various directions, as 

 streams of granules ; in these we have the foundation of the 

 patterns assumed by secondary deposit, and the source from which 

 it is derived. The elat^r cells in the young fruit of Marchantia 

 have been watched to trace the process, and the first thing seen is 

 a parietal border of protoplasm, enclosing starch granules, next 

 vacuoles appear, and then the granular protoplasm commences a 

 distinctly spiral movement, the starch disappears, and a homo- 

 geneous layer of cellulose is seen clinging spirally to the cellulose 

 case. 



Bordered open pores are best seen in young shoots of Conifers, 

 and commence by an infolding of the primordial utricle, then the 

 bordering cavity enlarges, and the pore canal becomes narrowed by 

 internal deposit. In most parenchym cells, Iodine shows ns that 

 the cellulose remains unaltered, but in other cases a change takes 

 place ; thus wood, bast, and vascular cells, and many unilaterally 

 thickened epidermis cells, lose their original soft condition, and 

 become hard ; Iodine tests now colour them yellow, and they are 

 dissolved in warm caustic potass, and also by nitric acid, the 

 cellulose has been converted into Lignin ; again, cells of the 

 Peridei-m and of cork tissue present the same reaction, but are not 

 dissolved by nitric acid, and moreover, they contain nitrogen. 

 Here the cellulose is changed into Suberin. 



Lignification commences at the exterior, but never entirely 



