688 MICKOSCOP1C STRUCTURE OF PHANEROGAMIC PLANTS 



demarcation becomes obvious in the form of an intermediate lamella , 

 at one time called ' intercellular substance,' and supposed to be a 

 distinct structure, but now shown to be the result merely of a differ- 

 ence in density or molecular structure of the cell-walls during their 

 thickening. This layer very frequently ultimately assumes a muci- 

 laginous character. Where cells have a rounded outline, it is 

 obvious that intercellular spaces must exist between them ; and as 

 the tissue develops, these spaces often increase greatly in size. They 

 are called schizoyenous if formed simply by the parting of cells from 

 one another ; lysigenous if resulting from the disappearance or 

 absorption of ceils. Recent observations have shown that the wall 

 of intercellular spaces is frequently clothed with a lining of proto- 

 plasm. There are many forms of fully developed cellular paren- 

 chyme, in which, in consequence of the loose aggregation of their 

 component cells, these may be readily isolated, so as to be prepared 



for separate examination without 

 the use of reagents which alter 

 their condition ; this is the case 

 with the pulp of ripe fruits, 

 such as the strawberry or currant 

 (the snowberry is a particularly 

 favourable subject for this kind 

 of examination), and with the 

 parenchyme of many fleshy leaves, 

 such as those of the carnation 

 (Dianthus caryophyttus) or the 

 London pride (Saxifraga wm- 

 brosa). Such cells usually con- 

 tain evident nuclei which are 

 turned brownish-yellow by iodine, 

 whilst their membrane is only 

 turned pale yellow, and in this 

 way the nucleus may be brought 

 into view when, as often happens, 

 it is not previously distinguish- 

 able. If a drop of the iodised 



.solution of chloride of zinc be subsequently added, the cell-membrane 

 becomes of a beautiful blue colour, whilst the nucleus and the granu- 

 lar protoplasm that surrounds it retain their brownish-yellow tint. 

 The use of dilute nitric or sulphuric acid, of alcohol, of syrup, or of 

 several other reagents, serves to bring into view the ' primordial ' or 

 parietal utricle, its contents being made to coagulate and shrink, so 

 that it detaches itself from the cellulose wall with which it is ordi- 

 narily in contact, and shrivels up within its cavity, as shown in 

 fig. 524. It would be a mistake, however, to regard this as a distinct 

 membrane ; for it is nothing else than the peripheral layer of proto- 

 plasm, naturally somewhat more dense than that which it includes, 

 but passing into it by insensible gradations. 



It is probable that all cells, at some stage or other of their 

 growth, exhibit, in a greater or less degree of intensity, that curious 

 movement of cijclosis which has been already described as occurring 



FIG. 527. Cubical parenchyme, 

 stellate cells, from petiole of 

 lutea. 



with 



