140 PRACTICAL BOTANY 



sisting of three layers, with depressed stomata : also the central 

 mesophyll, differentiated into palisade and spongy parenchyma. 



Sections should be cut from young leaves, from which evidence 

 may be obtained that the aqueous tissue is really the product of 

 periclinal division of the originally simple layer of dermatogen. 



The bodies known as cystoliths are the most prominent 

 deposits of calcium carbonate to be found in connection with cell- 

 walls. In order to see these mount transverse sections of a leaf 

 of Ficus elastica in water, and examine with a high power. 



Observe the layer of large clear cells underlying the superficial 

 layer of the epidermis of the upper surface of the leaf : here and 

 there one of these cells is seen to contain a botryoidal body sus- 

 pended by a stalk from the top of the cell ; this is a cystolith : it 

 consists of a mass of cellulose developed as an outgrowth from 

 the cell -wall, incrusted with calcium carbonate. 



Run in a drop of acetic acid : observe that the cystolith becomes 

 gradually transparent, and that an evolution of bubbles of gas 

 (C0 2 ) is taking place from it. 



When the calcium carbonate is all dissolved, a mass of cellulose 

 will be seen to remain, presenting both striation (perpendicular 

 to its margin) and stratification (parallel with its margin). Apply 

 tests for cellulose to this residue. 



II. Mechanical strengthening of leaves. The scleren- 

 chyma (stereom) has been above alluded to in the case of the 

 lamina of the Holly, and the phyllode of Acacia, as a tissue which 

 is useful in giving strength and rigidity to the other less firm 

 tissues : a further and striking example of the distribution of 

 sclerenchyrna so as to attain this end is seen in species of Hakea 

 for instance, in H. suaveoleus. Cut transverse sections of the 

 very rigid pinna, and, mounting as before, note that the oval 

 section is limited by a firm layer of epidermis, with here and 

 there stomata depressed below the general surface, and bounded 

 by two very small guard-cells, with a pair of small subsidiary 

 cells on either side : below this are two layers of palisade paren- 

 chyma, and centrally a colourless mass of parenchyma inclosing 

 several vascular bundles, and scattered sclerenchyinatous elements. 

 Note especially the large, sometimes branched, sclerenchymatous 

 cells, which extend from the epidermis to the central mass of 



