THE HIGHER CRYPTOGAMIA. 209 



arched. The longitudinal growth of the frond is also for- 

 warded by the production of septa which are parallel to the 

 fore and hind walls of the cell of the first degree, and are 

 turned towards the surfaces of the frond. From time to time, 

 however, the apical cell divides anew by longitudinal septa 

 at right angles to those just mentioned, and the end of the 

 young frond is by this means widened. Thenceforward 

 both forms of division continue to take place in the marginal 

 cells of the frond which adjoin the apical cell; but the 

 activity of division diminishes in a lateral direction, and 

 terminates far above the place of insertion of the frond. 

 That portion of the frond which is situated above the point 

 at which the multiplication of the marginal cells terminates, 

 becomes the blade of the frond, and the portion below that 

 point becomes the stem of the frond. The cell-succession 

 of the leafy portion of the frond therefore much resembles 

 that of the flat stem of the Marchantieae and Riccieae ; but 

 there is invariably one cell only of the first degree ; not 

 two. 



The formation of the pinnae of the frond in the species of 

 Pteris, as in the rest of the Polypocliaceae, is the result of a 

 true bifurcation of the apical punctum vegetationis. This 

 formation commences with the division of the apical cell by 

 a septum coinciding with the median line of the frond, and 

 perpendicular to its surfaces. Each daughter-cell is divided 

 by a septum almost parallel to the longitudinal axis of the 

 frond (PI. XXIX, fig. 3). This latter division occurs either 

 immediately, or after the previous formation of septa which 

 are inclined to the surfaces of the frond, and contribute to 

 its longitudinal growth. The three-sided cell on the right 

 and on the left of each of the tw r o pairs of cells which 

 occupy the middle of the fore edge of the frond, becomes 

 the seat of fresh cell-multiplication, and is the cell of the 

 first degree of a pinna of the frond. Each of the new 

 shoots is alternately more strongly developed, thus changing 

 the direction of the bifurcation to the right or to the left. 

 The weaker one is pushed on one side so as to appear to be 

 lateral. The continual change in the direction of the less 

 vigorous bifurcations causes the feather-like form of the 

 frond, whose segments (as is well known) are in no species 



14 



