86 THE PLANT CELL. 



the cell-cavity from the outer cell-wall. On the addition of 

 acetic acid the carbonate of lime dissolves with effervescence, 

 leaving the core of cellulose intact. It is worthy of note in 

 this connection that the epidermis of the leaf of Ficus is three- 

 layered, a somewhat unusual occurrence. 



D. Idioblasts. 



Certain isolated cells occur at times in various parts of a 

 plant which have the specific function of secreting or excreting 

 substances detectible by the employment of special tests ; such 

 cells are known as idioblasts. One of the commonest forms is 

 the tannin-celJ, which is found in the cortex of such plants as 

 Pinus and Quercus. In the former tannin-cells are recognised by 

 treating a fresh transverse section of the stem with a dilute 

 solution of perchloride of iron (FeClg), when these cells turn black, 

 owing to formation of tannate of iron. 



Another form of idioblast occurs in the petiole of the leaf of 

 Nymphcea. In this case large stellate cells are found at the 

 points of junction of the numerous strands of cells composing the 

 ground-tissue of the petiole, these stellate cells having walls 

 which are characterised by the presence on them of small pro- 

 jections formed of oxalate of lime (see Fig. 71, c). Their 

 function is not obvious. 



E. Laticiferous Cells and Vessels. 



These elements are characterised by the presence in them of 

 a secretion known as latex, a thick or thin milky fluid composed 

 of a mixture of gums, proteids, and resins, which at times 

 coagulates spontaneously, or on heating (india-rubber). 



The vessels in which this latex occurs may be seen in 

 transverse and longitudinal sections of Ewphwhia stem, or in the 

 stem of Ficus elastica ; in longitudinal sections the vessels are 

 seen to be branched, and communicate here and there by means 

 of short lateral passages. They are formed by the early develop- 

 ment of elongated passages which arise by the lengthening and 

 branching of prosenchymatous cells, and these, by their further 

 differentiation, give rise to a system of branched canals in the 

 cortex of the stem. The latex is formed from the protoplasm 



