232 HOFMEISTER, ON 



cessively to one of the lateral surfaces. As far as can be 

 judged from numerous observations, the succession of these 

 septa one after another is to the right hand, more seldom 

 to the left, but ahvays coincident with the spiral in which 

 the fronds are arranged. 



There is yet a second point in which the relation of the 

 apical cell to its daughter-cells is affected by the frond- 

 spiral. The apical aspect of the top cell of old speci- 

 mens of Aspidium filix-mas is very rarely that of an equi- 

 lateral triangle. One of the sides is usually considerably 

 shorter than the two others, which latter are nearly of equal 

 length. The outline of the apical surface is normally that 

 of an isosceles triangle. Deviations from this form may 

 be easily traced to the disturbances caused in the older 

 lateral surfaces of the apical cell by the growth of the 

 adjoining secondary cells. The one side of the triangle is 

 formed by the upper edge of the youngest side- wall of the 

 terminal cell, and the other side by that of the oldest side- 

 wall of the same cell. The base is formed by the side-wall 

 intermediate in age between the oldest and the youngest. 



The relation of: the length of this base to the younger of 

 the two sides is in most cases a definite one. The follow- 

 ing series of measurements will show this. The younger 

 of the two longer side-walls of the apical cell is the one 

 always measured. Some of the measurements were made 

 on the apical cells of buds which had been separated by a 

 transverse section from the older portion of the stem, and 

 simply cleansed from the adherent mucilage and scales. 

 The greater part of the measurements, however, were made 

 on the transparent membrane formed by the free outer 

 walls of the superficial cells of the bud. These walls have 

 a much stronger consistence than those of the inner tissue 

 of the bud. After a little practice with the microscope it 

 is not difficult to scrape out from the inside of the terminal 

 bud the mass of internal parenchyma, consisting of delicate 

 cell-walls and cell-contents, so as to leave the outer walls 

 in the form of a connected, slightly arched membrane 

 (the epidermis, improperly so called, of the young portions 

 of the plant). The lines of contact of the cell-walls 

 which have been attached to this membrane are most dis- 



