238 H0FME1STER, ON 



the frequency of the correspondence in form of the upper 

 surface of the cell of the first degree with the frond 

 arrangement, as well as the rarity of the deviations from 

 this form. Moreover, the observation confirms the conse- 

 quent backward curvature of the lines uniting the project- 

 ing angles (which are turned to the same side) of the dif- 

 ferent courses of the successive cells of the second degree 

 around the axis of the stem ; which lines represent three 

 similar turns of the frond-spiral. The above opinion is 

 further supported by the fact, that the expansion and dis- 

 placement supposed to occur in the apical cell, must neces- 

 sarily follow from the enlargement and multiplication, 

 progressing gradually from the older to the younger ones, 

 of the cells of the second degree. The marginal angles 

 of the lateral surfaces of the cell of the first degree, must, 

 in the direction of the ascending spiral which represents the 

 course of the divisions, become more acute at the fore 

 edges, and more obtuse at the hinder ones, if as observa- 

 tion proves the multiplication of the older cells of the 

 second degree in the direction of a tangent to the stem is 

 more active than that of the youngest cells. In this process 

 the apical cell may be looked upon as to a certain extent 

 passive. 



The supposition of a high degree of expansive and for- 

 mative power in the walls of the young cells of a portion 

 of a plant in process of development, is indispensable for the 

 purpose of explaining the change in the position and form 

 of the individual cells, which is caused by the growth of the 

 entire portion of the plant, and by the influence of the expan- 

 sion (and the multiplication of the older cells and masses of 

 cellular tissue) upon the younger ones, and conversely. In 

 the terminal bud of ferns expansion and multiplication of the 

 secondary cells, and of the groups of cells produced by their 

 divisions, advance in an ascending spiral from below up- 

 wards. In the neighbourhood of the apical cell this expan- 

 sion occurs at an early period (and is consequently more 

 advanced and productive of greater results) in the oldest 

 wall which forms the base of the upper surface of the cell, and 

 in the next oldest, whose margin forms the penultimate side 

 of that surface. The growth of the apical cell, which, between 



