140 HOKMEISTER, ON 



pendicular to the surface of the leaf. The edge formed by 

 the contact of this latter septum with the upper side wall 

 of the mother-cell coincides exactly with the line in which 

 the membrane just produced in the apical cell cuts the 

 boundary wall of the cells of the first and second degree. 

 The septum in question forms a right angle with the side 

 walls of the cell of the second degree ; its direction is, 

 therefore, exactly the same as that of the septum by which 

 the apical cell was contemporaneously divided. The inner 

 of the cells into which the second youngest cell of the 

 second degree is divided has a rather long, rectangular, 

 basal surface. Both the cells of the third degree, which are 

 produced by the division of the cells of the second degree, 

 are soon divided, by longitudinal septa parallel to the side 

 walls, into equal parts, whose basal surfaces are almost 

 exactly square (PL XVIII, fig. 1). A similar process takes 

 place upon each further division of the apical cell of the 

 leaf. All the cells of the edge of the leaf which lie in the 

 course of the prolongation of the line of direction of the 

 newly produced septum of the apical cell divide, almost 

 contemporaneously, with the apical cell, by septa whose 

 direction coincides with that of the above-mentioned line 

 (PI. XVIII, fig. l b ). The result of these processes is that 

 in all species of Sphagnum the young leaf, with the excep- 

 tion of its margins, appears divided into regular squares. 

 With the exception of its edge, which appears composed of 

 somewhat elongated cells, the entire surface of the leaf con- 

 sists of cells whose basal outline is square, and each four 

 of which are in contact at their edges. It is only occa- 

 sionally that in these divisions one cell is passed over, and 

 then one cell of the interior of the leaf is twice as wide as 

 its neighbours, and its basal surface has the form of a paral- 

 lelogram (PL XVIII, fig. 1 ; see one of the cells of the fifth 

 of the oblique rows to the left).* 



It is self-evident that, by the repeated division of the 

 cells, the lower, older portion of the leaf increases consider- 



* Prom the above-mentioned processes by which the chess-board-like 

 arrangement of the cells of the young leaf is produced, Nageli concludes that 

 the division of one cell has a manifest effect upon the neighbouring cell, and 

 causes the division of the latter in the same direction ('Pflanzen-phys. Unters.,' 

 i, p. 78). On the other hand, I find in these facts a new ground for the con- 



