STRUCTURE OF LEAF 331 



through the centre of the stem is a mass of tissue (ax. b) 

 distinguished by its small, thin-walled cells, and constituting 

 the axial bundle. 



The leaves (B) are shaped like a spear-head, pointed 

 distally, and attached proximally by a broad base to the 

 stem. The axial portion (B and E, md. r., c. /) consists of 

 several layers of somewhat elongated cells and is called the 

 midrib : the lateral portions (E and F : c, I 1 } are formed of a 

 single layer of short cells. Thus the leaf has, for the most 

 part, the character of a superficial aggregate. The cells 

 contain oval chromatophores (F, chr). 



The rhizoids (c and D, rh) are linear aggregates, being 

 formed of elongated cells, devoid of chlorophyll, arranged, 

 end to end. 



In the terminal bud the leaves, as in Nitella (pp. 206 and 

 208), arch over the growing point of the stem, which in this 

 case also is formed of a single apical cell (c and G, ap. c). 

 But in correspondence with the increased complexity of the 

 plant the apical cell is not a hemisphere from which new 

 segments are cut off parallel to its flat base, but has the form 

 (H) of an inverted, three-sided pyramid or tetrahedron, the 

 rounded base of which (a be} forms the apex of the stem 

 while segments (seg. c} are cut off from each of its three 

 triangular sides in succession. 



The best way to understand the apical growth of a moss 

 is to cut a tetrahedron with rounded base out of a carrot or 

 turnip : this represents the apical cell (H) : then cut off a 

 slice parallel to the side abd^ a second parallel to bed, and a 

 third parallel to acd : these represent three successively 

 formed segments. Now imagine that after every division 

 the tetrahedron grows to its original size, and a very fair 

 notion will be obtained of the way in which the successive 

 segments of the moss-stem are formed by the fission in three 



