38 PRACTICAL BOTANY 



may thus be easily recognized (see p. 26, reactions of corky 

 walls). 



The granular protoplasmic contents of these cells (brown, 

 with chlor-zinc-iodine) are not plentiful, but form a thin layer 

 lining the somewhat rounded cell-cavity, Chlorophyll- grains 

 are to be found in these cells : this point is to be noted, since 

 in the steins of many plants chlorophyll is absent from the 

 epidermal cells. 



The cells surrounding the bases of the hairs, as well as the 

 underlying tissue, show a luxuriant growth : in fact the hairs are 

 each seated at the apex of an emergence. 



2. In the collenchyma the protoplasmic body resembles 

 that of the epidermis : chlorophyll-grains are numerous. The 

 cell-walls also are highly refractive, and stain blue with chlor- 

 zinc-iodine (cellulose) ; they are specially thickened at the 

 angles, where three or more cells meet ; in the thickened mass 

 the lines of stratification are well seen. Note that the collen- 

 chyma is not continuous below the stomata ; also that there is 

 no sharp internal limit to the collenchyma, but it merges 

 gradually into 



3. The thin-walled cortical parenchyma, which differs from 

 the preceding (a) in the thinness of its walls, (b] its less copious 

 cell-contents, (c) the larger size of the cell-cavity, (d) the pres- 

 ence of intercellular spaces, which result from the splitting of 

 the cell-walls at the points where three or more walls meet ; in 

 the living state they are filled with air, and even in specimens 

 which have been treated with alcohol, air-bubbles may still be 

 found entangled in them. 



Observe carefully the resin-passages, which occur in the 

 cortical parenchyma. The resin, being soluble in alcohol, has 

 been removed. They are intercellular spaces, formed by the 

 splitting of cell-walls. The cavity thus formed is surrounded 

 by small, thin-walled epithelium, the cells of which divide both 

 radially and tangentially as regards the passage. 



Note that in the epidermis, collenchyma, and thin-walled 

 parenchyma of the cortex, there occur divisions of the cells in 

 a radial direction. Compare the girth of the stem at the upper 

 with that at the lower part of the plant, or that of a young plant 



