42 HOFMEISTER, ON 



parallel to the surface of the stem are only found in the one 

 innermost cell adjoining the longitudinal axis of the shoot. 

 After the first division of this kind, the formation of a 

 horizontal septum is repeated twice in each of the under 

 cells. The middle line of the stem consequently consists 

 of two parallel rows of four tabular cells one above another ; 

 the remaining part of the stern is a single superficies of 

 cells. Both the inner pairs of cells of the mid-rib which 

 protrudes from the lower side, divide by longitudinal septa 

 at right angles to the surface of the stem ; they are, there- 

 fore, about half the size of the adjoining cells of the upper 

 and under sides (PL V, fig. 29). Sometimes the cells of the 

 underside of the mid-rib follow in this division. The under 

 side of the fore edge of the mid-rib sends out numerous bicel- 

 lular hairs with a swollen terminal cell, and which, bending 

 themselves upwards, enclose to a certain extent the growing 

 end of the stem. The entire under surface of the stem 

 sends out rootlets, which are especially numerous on the 

 mid-rib and the side edges. The young cells exhibit a 

 nucleus with transparent fluid contents, which is freely 

 suspended in the slightly granular cell-sap. The nucleus 

 lasts for a long time, and, by means of the chlorophyll ad- 

 herent to its exterior, it is perceptible even in older cells, 

 where its contents refract the transmitted light much in 

 the same manner as the fluid contents of the cell. The 

 chlorophyll-granules of Metzgeria are amongst the smallest 

 in the vegetable kingdom. 



Adventitious shoots are often developed from individual 

 cells of the edge or of the under side of the mid-rib 

 of plants growing in dry situations. The cell-multiplica- 

 tion in such shoots, which is very easy to observe, takes place 

 in precisely the same manner as it does in growing primary 

 shoots (PI. V, fig. 26). Vigorous adventitious shoots, 

 whilst still very young, form a mid-rib in the same manner 

 as the growing primary shoot, which mid-rib, by the divi- 

 sion of the cells lying between the first cell of the adventi- 

 tious shoot and the mid-rib of the primary shoot, is not 

 unfrequently prolonged backwards to the mid-rib of the 

 primary shoot. Sometimes, in unhealthy specimens, the 

 formation of cells of the third order is entirely suppressed ; 



