THE HIGHER CRYPTOGAMIA. .23 



three days in a plant kept moist upon the stage of the 

 microscope. If the number of the apical cells of the plant 

 increases in a lateral direction by the division of the exist- 

 ing cells, the like process immediately follows in all the 

 newly formed cells : in the entire row of apical cells 

 division takes place continually by septa inclined either to 

 the upper or the under surface of the plant. The newly 

 formed cells of the second order divide, very shortly after- 

 wards, by septa almost parallel to the surfaces of the plant 

 (PL IV, fig. 13). By this means the number of layers of 

 cells increases, and with it the thickness of the plant. This 

 increase is only slight at first ; it does not seem that the 

 process is repeated whilst the plant is quite young, or 

 that either longitudinal or transverse sections exhibit more 

 than four layers of cells. Very frequently those cells only 

 divide which are turned towards one (? the lower) surface ; 

 the plant then consists, in the direction of its thickness, 

 of three layers of cells (PI. IV, fig. 16). By this time 

 certain of the superficial cells of the plant are divided once 

 or twice by a septum perpendicular to the surface, which 

 cells consequently appear twice or three times as small 

 as those in the interior of the tissue (PI. IV, fig. 13). This 

 is afterwards the normal condition. By the penetration 

 of the first rootlets into the ground, the young plant 

 is set erect. It retains this position only a short time. 

 The expansion which immediately commences in the 

 the cells of its lower portion is not uniform ; the cells of one 

 surface expand far less in a longitudinal direction than 

 those of the other (PL IV, fig. 13) : hence it follows, that 

 the apex of the plant becomes more and more inclined to- 

 wards the less expanded side of the basal cell, i. e., .the 

 future under surface of the plant ; so that the direction of 

 the growth of the young Pellia is parallel to the surface of 

 the soil beneath it. Individual cells of the under side, 

 especially those lying in the median line, grow out into 

 long rootlets, which penetrate deeply into the ground. 

 The rootlets originate in the following manner : at a certain 

 point, usually, exactly in the middle of the outer surface of 

 one of these cells, the membrane grows out into a strongly 

 developed point, which shortly leads to the formation of a 



