70 V. CELL-WALL, AND CELL CONTENTS. 



resulting brownish colour. With polarised light the sphaerites of 

 inuline show a black Maltese cross on a bright ground, just as 

 do starch grains. 



Calcium Phosphate. The sphaerites of dahlia tubers are not 

 always of inuline. In tubers preserved in alcohol, bulky sphaero- 

 crystals of calcium phosphate are often deposited. They have 

 the same form, but usually smaller size than those of inuline, 

 and are slowly dissolved in water if the preparation be ex- 

 amined therein, their contour remaining to the last, while their 

 refractive power continually decreases. The sphaerites of cal- 

 cium phosphate are either without clear lamination and distinct 

 nucleus, or they show an amorphous nucleus, which is surrounded 

 by a shell of acicular crystals. In the latter form the nucleus of 

 the sphaero-crystal is deeply stained by carmine solution ; in the 

 former the sphaero-crystal is stained throughout its entire mass, 

 and in this manner can again be made visible after it has almost 

 disappeared through the action of water. In the one case the 

 organic substance is confined to the nucleus ; in the other it is 

 pretty uniformly distributed through the entire mass of the 

 sphaero-crystal. These sphaerites rapidly disappear when acted 

 upon by nitric acid. Concentrated sulphuric acid run in under 

 the cover-glass turns them brown at once, and then quickly 

 changes them into nests of crystals of gypsum (sulphate of lime), 

 while the sphaero-crystals of inuline, on the other hand, appear 

 unchanged. 



If the dahlia tubers do not show sphaero-crystals of calcium 

 phosphate, they can be readily found in other plants. They are 

 certain to be found in the ground-tissue of the fleshy Euphorbias, 

 cultivated in plant-houses, such as E. Caput- Medusa, when this 

 has been laid in spirit. They are found very beautifully in the 

 pith of the inflorescence of E. helioscopia (the sun -spurge), when 

 this is preserved in spirit. 



Reactions of Tannin. In order to demonstrate the tannin- 

 reaction upon a typical object, we turn to the gall-apple or 

 oak-gall, as found upon the leaves of our oaks. These gall- 

 apples are due to the puncture of the oak-gall insect (Cynips 

 querci), which lays an egg in the punctured tissue. We halve 

 such a gall-apple while still young, and find on delicate radial 

 sections taken from it that the interior hollow, occupied by the 

 larva of the Cynips, is surrounded by a sheath, which consists of 

 iso-diametric, rounded cells. These contain usually abundant 



