12 THE CELL WALL 



lively little detailed iiit'ormatiuii is available regarding the 

 ontogeny of pits. Possibly their usual paired character is asso- 

 ciated with the fact that at least in living cells, the pit membrane 

 is penetrated by plasmodesmata which thus may determine the 

 opposite position of the pits. At any event it is clear that pit-pairs 

 arise on the primary pit fields of meristematic cells. These pit 

 fields which are defined (cf. Glossary of Terms Used in Describ- 

 ing Woods, p. 4) as thinner areas of "the intercellular layer and 

 primary walls" are observable in cambial initials as well as in the 

 so-called i)rimordial meristem of the shoot apex of certain seed 

 plants (cf. Foster, 1938, 1939a, in "References" to Exercise III). 

 From the standpoint of function, pits are believed to facilitate 

 the process of diffusion between adjacent cells. 



Pit-pairs may be conveniently classified under four major 

 types, viz. : 



1. Simple pit-pairs, which are typical of cells which retain a 

 protoplast throughout their functional life, are particularly well 

 developed in parenchyma cells. In face view, the aperture ap- 

 pears as a circular, elliptical or even irregular area. In macer- 

 ated tissue,^ simple pit-pairs in this view appear as refractive red 

 points of light. A recognition of this optical characteristic will 

 help to distinguish simple i)its from jiarticles of i)rot<)plasm or 

 other substances lying free in the cell lumen. In sectional view, 

 the cavity of each member of the pit-pair is usually of equal 

 diameter throughout and there is no overarching rim or border 

 produced by the adjoining secondary wall. 



Ohtaiii ii pi-eparation of macerated secondary xylem of the 

 .stem of the trumpet-creeper (Tccrnna radicdiis) and examine it 

 undei- l(/)r magnification noting the numerous wood parenchyma 

 and xylem-ray parenchyma cells. These cells are box-like in 

 form and occur singly or in groups depeiuliiig upon the extent 

 to which the xylem has been macerated. Study a coiniected 

 group of parenchyma cells under lii(/Jt niannification and investi- 

 gate the size, structure and position of the simple pit-i)airs as seen 

 in surface and sectional vieAvs. In nil studies of this kind, it is 

 e.ssentijil to sccni-e ci-itical illuuiination and to use the fine adjust- 

 ment of the mici-oscope constantly. 



' Cf. Appoiulix, J)]). 140-141 for tlie t('(lmi(|ue of niacoiatiiiK i)iant tissue. 



