THE HIGHER CRYPTOGAM TA. 273 



multiplication extending in many cases even beyond the 

 twelfth internode. In the cells of the pith only a single 

 transverse division occurs. Prom the fact that the thick- 

 ness of the shoot is in such manifest connexion with the 

 period of the occurrence of the transverse division of the 

 pith-cells, it may perhaps be concluded that the immediate 

 operation of the outer air upon the tissue of the growing 

 stem has an especial effect in promoting transverse division 

 in the cells. 



The formation of the epidermis of the young stem-joint 

 is contemporaneous with its longitudinal extension, with its 

 appearance above ground, and with the formation of nu- 

 merous chlorophyll bodies in the cells of its circumference. 

 All the cells of the outer surface divide twice by transverse 

 septa, then by longitudinal septa, and lastly by septa paral- 

 lel to the outer surface. A double layer of cells is thus 

 produced, enclosing the circumference of the stem, the cells 

 being one-eighth the size and eight times as numerous as 

 those of the next inner layer. The outermost are trans- 

 formed into the epidermis ; every second cell of the epider- 

 mis of the above-ground shoots becomes the mother-cell of 

 two stomatal cells ; these as well as the tabular cells of the 

 epidermis exhibit upon the outer surface very regular pro- 

 jections the form of which is constant for each species (PI. 

 XXXVI, fig. 2). These projections contain more siliceous 

 matter than any other part of the stem. 



The differentiation of the vascular bundles from the 

 surrounding tissue commences a short time before the for- 

 mation of the epidermis. The first commencement of the 

 vascular bundle consists in the appearance of annular fibres 

 in a vertical row of cells the position of which answers 

 exactly to one of the tips of the next higher leaf. From 

 five to six of these annular fibres occur in each cell (PI. 

 XXXV, fig. 13). A plane passing through the middle of 

 the tip of the leaf cuts the string of cells of the stem-joint 

 which bears the leaf, in each of which cells annular fibres 

 are formed. 



The horizontal septa which separate the ring-bearing 

 cells from one another are very soon absorbed, and a circle 

 of annular vessels traversing the entire length of the stem- 



18 



