STRUCTURE.] 



PITS PORES. 



15 



readily, with a magnifying power of 350 diameters. But it is 

 by no means to thin transparent tissue that these passages 

 are confined ; they are universally present in the sides of the 

 thickest sided tissue, where they form minute cul-de-sacs, often 

 branched, and always opening into the interior of the cell. 

 They may be readily found in the gritty tissue of the pear 

 (fig. 2. a), the stone of the plum b, and the compact albumen 

 of seeds. Fig. 2. c represents them in the albumen of Alstro- 

 meria, where they are about T 3Vo- f an inch in diameter. 





By what power the sedimentary matter, left on the sides of 

 such tissue as this, is prevented from choking up the pits, is 

 at present unknown. 



It is, no doubt, very common for the pits of the membrane 

 of one cell to be placed exactly opposite those of the next cell, 

 as is seen in the irregular half- gelatinous tissue of Cereus 

 grandiflorus (see Plate II. fig. 1. a a), so that it may be 

 supposed that they are passages to allow of permeation from 

 one cell to another ; but this arrangement is by no means 

 uniform (see same fig. &).* 



The absence of pores from vegetable membrane has been 

 so generally admitted, that it would have seemed needless 

 to insist upon the erroneous notions of the old anatomists, 

 had not two recent observers, Harting and Mulder, revived 

 the question by asserting that, almost universally, the yet 



* For the supposed chemical difference between elementary membrane and 

 fibre, see Book II. Chapter I. 



