PITS. PLASMODESMA. STRATIFICATION 45 



probably exhibits peculiarities of structure that render it specially 

 permeable. The mere fact that the plasmatic membrane does not 

 induce growth in thickness of the cell-wall over certain areas proves 

 that it is in some respects different, at these points, as compared with 

 the remainder of the ectoplast. In most plants the closing-membranes 

 of simple pits are perforated by minute pores, which are traversed by 

 protoplasmic connecting threads or plasmodesma. 1 * Not infrequently 

 plasmodesma are developed also in connection with the thickened 

 regions of the wall. These plasmodesma provide the only means of 

 direct communication between adjacent protoplasts. In certain cases 

 they help to expedite diffusion, but their principal function undoubtedly 

 consists in the transmission of stimuli from cell to cell, for which 

 reason the detailed discussion of these structures must be postponed 

 until a later chapter. (Cf. intercellular transmission of stimuli, Ch. 

 XIII. II. A.) The bordered pit, a peculiar modification of the ordinary 

 or simple pit, is a characteristic and physiologically important feature 

 of certain water-conducting elements, and for this reason is most 

 conveniently discussed in connection with the conducting system 

 (Ch. VII. II. A, 1 and 2). Generally speaking, in fact, the character- 

 istic mode of thickening of the cell-wall is so closely correlated with 

 the special functions of the cell or tissue to which it belongs, that 

 this subject cannot properly be dealt with in a general account of the 

 cell. It may, however, at once be stated that the great diversity 

 which prevails with regard to the details of cell-wall thickening is 

 partly a matter of variation of design [cf. p. 6]. 



The internal differentiation of secondarily thickened cell-walls 19 

 occasionally possesses a definite physiological significance, but more 

 often merely represents a consequence of the special mode of develop- 

 ment of the membrane. 



Stratification, that is the differentiation of a number of concentric 

 layers differing among one another in respect of refractive index, is an 

 almost universal feature of thickened cell-walls. If such a stratified 

 wall is caused to swell by treatment with suitable reagents, each 

 individual stratum is often found to be composed, in its turn, of a 

 number of delicate lamellae. Very frequently a thickened wall may 

 be seen to consist of several stratified layers or complex strata, which 

 differ from one another in chemical composition as well as in their 

 optical behaviour; the so-called primary, secondary and tertiary thickening 

 layers represent strata of this nature. As a rule the secondary layers 

 form a massive stratum, while the tertiary layers are represented by 

 a very thin and often highly refractive, pellicle [sometimes termed the 

 "internal" or "limiting pellicle" {Grmzhautcheri)]. 



According to the view first put forward by Niigeli, stratification, 



