AIR SPACES AND SECRETIONS. 



77 



alcohol (not water, because it acts on the mucilage). Mount 

 a section in dilute alcohol or 

 glycerin. Smaller cells 

 densely filled with starch 

 constitutes the greatest pro- 

 portion of the section. Scat- 

 tered irregularly among 

 these are numerous larger 

 rounded sacs containing a 

 transparent substance the 

 mucilage sacs. 



Add a drop of iodine so- 

 lution to a fresh section. 

 Most of the cells turn dark 

 (starch), but the mucilage 

 sacs remain as before. In 

 the large central area 

 groups of yellow-stained 

 wood bundles are visible, 

 while in the cortical layer 

 groups of similarly stained 

 bast fibres are seen. A dis- 

 tinct circle of small cells, 

 free from starch, divides the 

 cortical layer from the 

 larger central area. It is the cambium zone. 



Remove nearly all of the liquid from the iodine section and 

 add a drop or two of sulphuric acid (diluted with one-fourth 

 water). The mucilage is stained a deep brown and the walls 

 blue (cellulose). Under favorable conditions delicate con- 

 centric lines may be seen in the mucilage. 



Soften a piece of Elm bark and examine cross-sections in the 

 same manner as in the case of marsh-mallow root. It has large 

 and numerous sacs. 



Fig. 47. Cross-section of Althaea root, sch, 

 mucilage sacs ; c, cambium ; b, bast fibres ; 

 Ib, a bundle of wood vessels and fibres ; m, 

 medullary rays (Tschirch). 



MUCILAGE AND RAPHIDE SACS IN SQUILL BULB. 



Crystals are of such general occurrence in widely different 

 orders of the higher plants that there are perhaps none in 

 which they may not be detected. They have been found in 

 nearly all parts of the vegetable structure, more commonly in 

 the interior of parenchyma cells, sometimes in specialized 

 crystal cells. 



They occur either singly or in groups. The most common 

 forms are the octahedron and the prism. Sometimes the crys- 

 tals are much elongated and pointed, like a needle, and are then 

 known as raphides (from raphis, a needle). These are gener- 



