248 BOTANY 



ward into the heavily-thickened basal portion of the fibre. This 

 would seem to suggest that as the membranes grow upward their 

 lower extensions gradually become transformed into cell-wall 

 lamellae. 



In the more highly differentiated portions of the fibre, there 

 are in addition to the lamellae, which extend upward into the elon- 

 gating tip of the cell, a series of concentric layers (Fig. 83, 4), which 

 form more or less elongated compartments. The upper and lower 

 extremities of these compartments, which appear as semi-circular 

 walls occluding the lumen, are referred to by different authors as 

 " caps.'' The innermost of these unmodified lamellae is an un- 

 modified membrane. This is in marked contrast to the condition 

 exhibited in the younger portion of the fibre, in which the upper 

 extremity of the compartment terminates in a series of tenuous 

 hyaline membranes, which are in direct continuity with the 

 lamellae in the lower portion of the compartment. Such facts as 

 these suggest that whereas the outer cell wall lamellae are differen- 

 tiated from delicate membranes which grow upward from base 

 to apex of the fibre, the lamellae of the included compartments 

 are produced by membranes of considerably restricted longi- 

 tudinal growth. There is direct continuity of protoplasm through 

 the caps, which are perforated by a terminal pore, and the terminal 

 portions of the unmodified membranes in the upper portion of the 

 fibre have apertures, so that there is complete continuity of the 

 protoplasm from one end of the fibre to the other. 



A detailed study of the fibres at different stages of enlargement 

 and differentiation shows that each of the successive hyaline 

 membranes arises from a previously-formed one. The new mem- 

 brane arises as an ingrowth from the basal portion of the primary 

 membrane and grows upward towards the elongating tip of the 

 young fibre. After the secondary membrane has attained a con- 

 siderable longitudinal extension, a tertiary membrane now arises 

 from its basal portion, and as this membrane becomes vertically 

 extended, a quaternary membrane arises in its turn from the basal 

 portion (Fig. 83, 3). During the later stages of the elongation of the 

 fibre, the successively-formed membranes grow upward towards 

 the apex of the cell, and as they do so, their basal extensions 



