OP SAP IN PLANTS. 13 



cortical layers, and had become more or less diffused from thence 

 into the surrounding parenchyma of the rind. The large vessels 

 of the stem just mentioned are of so great diameter that a hair 

 may be easily passed more than an inch down them. When the 

 stem was cut up, some of the fluid almost always escaped from 

 the wounded sap-bearing cells of the circumference and pene- 

 trated into the interior, as also in the garden balsam, where 

 the size and transparency of the tracheae invite such experi- 

 ments, giving an appearance as if these canals were not really 

 air-vessels but sap-conduits. Disregarding the fact that, in my 

 experiments, their walls and their rather fluid contents never 

 exhibited a blue colour, the original presence of air in them may 

 be easily directly demonstrated. For if a stem held the wrong 

 end upwards is cut across and the w^ound immediately dipped 

 in a drop of thick solution of gum, there is no difficulty in seeing 

 bubbles of air emerge slowly from these tubes and inflate the dense 

 fluid, when the stem is squeezed from the root upwards toward 

 the w^ound. The ascent of bubbles of air begins directly, even 

 when the pressure is commenced six inches below the wound, 

 and it continues without interruption if the pressure is advanced 

 slowly upwards to the cut surface. To conclude from the wholly 

 accidental presence of water in these tubes, that they are nor- 

 mally conductors of liquid, is like deducing from the presence of 

 blood in the air-passages of a decapitated animal, that the lungs 

 are naturally intended for that fluid. While the smaller vessels 

 of the stem are usually spiral vessels, the structure of the larger 

 is very well worthy of notice, for their walls look as if formed of 

 small, nearly quadrangular frameworks, over which a delicate, 

 closely spotted membrane appears stretched. The petiole, like 

 the stem, is very rich in starch-bearing parenchyma ; it is coated 

 by a rind, the partly elongated cells of which (but not the liber 

 cells) become deep blue ; the internal tissue exhibited very few 

 blue points, and the isolated spiral vessel bundles, in which air 

 was clearly seen under the microscope, bore no share in the 

 colouring. 



Rhapis flabeUiformis, Ait. (Palmaceae). — The plant, a specimen 

 three feet high, was penetrated throughout all parts by the fluid 

 in ten days after watering. — Stem : a transverse section, at least 

 in the upper part of the plant, displayed this enclosed in the 



