OF SAP IN PLANTS. 15 



cortical layer ; in the interne-dial parts of the stem the cortical 

 layer is contracted further inwards, so that there is not so sharp 

 a line of demarcation there. The points displaying the blue 

 reaction were among isolated roundish cells which occur inter- 

 nally and externally in the neighbourhood of the vascular 

 bundles. In longitudinal sections the blue points appeared in 

 an irregular band passing transversely through the node. In 

 buds examined four weeks after the watering (they had been 

 developed during this period),, blue colouring could be detected 

 inside the elongated cells of the calyx and the stigma, and inside 

 the minute parenchymatous cells of the (still white) petals and 

 of the connective, while the delicate spiral vessels, even in the 

 youngest, still imperfect organs, never displayed any colouring. 

 It is worthy of notice that the epidermis of the petals opposes 

 such a resistance to the penetration of the salt of iron, that no 

 reaction occurs even when the entire petal is dipped in sulphate 

 of iron ; hence it is necessary to tear the epidermis carefully 

 away to admit the sulphate of iron to the parenchyma when we 

 wish to bring out the reaction. 



Commelina pubescens. The presence of the ferrocyanide could 

 be shown in the cells of the stem and leaves five days after the 

 watering. 



Commelina clandestina, Mart. Here also the absorption could 

 be certainly demonstrated after a short period. 



Commelina angustifolia, Michx. Detected certainly after 

 four days. 



Commelina tuber osa. The absorption demonstrable in two 

 days, both inside longish cells and in isolated large cells of the 

 central parenchyma. 



Gladiolus psittacinus, .Hook. (Iridaceae). Here again it was 

 easy to trace the salt many inches up the stem, and above all in 

 the elongated cells, in which it was very uniformly diffused, and 

 which are in part situated close to the air-bearing spiral and 

 annular vessels, and in part at a distance from these and near 

 the inner and outer epidermis of the separate leafy sheaths, of 

 which the stem is almost entirely composed. The inner and 

 softer leaf-sheaths of the stem reacted most strongly and conse- 

 quently appeared to convey the sap most actively. These plants 

 must contain a salt of peroxide of iron as a natural constituent, 



