406 MR. SPENCER ON CIRCULATION AND 
roots there exists no adequate supply of moisture. A state of capillary tension must 
result—a tendency of the liquid to pass into the leaves, resisted below by liquid cohesion, 
Now, had the vessels impermeable coats, only their upper extremities would under these 
conditions be slowly emptied. But their coats, in common with all the surrounding 
tissues, are permeable by air. Hence, under this state of capillary tension, air will 
enter ; and as the upper ends of the tubes, being both smaller in diameter and less porous 
than the lower, will retain the liquids with greater tenacity, the air will enter tlie wider 
and more porous tubes below—the ducts of the stem and branches. Thus the entrance 
of air no more proves that these ducts are not sap-carriers, than does the emptiness of 
tropical river-beds in the dry season prove that they are not channels for water. There 
is, however, a difficulty which seems more serious. It is said that air, when present in 
these minute canals, must be a great obstacle to the movement of sap through them 
The investigations of Jamin. have shown. that bubbles in a capillary tube resist the 
passage of liquid, and that their resistance becomes very great when the bubbles are 
numerous—reaching, in some experiments, as much as three atmospheres. Nevertheless 
the inference that any such resistance is offered by the air-bubbles in the vessels of a 
plant, is, I think, an erroneous one. What happens in a capillary tube having im- 
pervious sides, with which these experiments were made, will by no means happen ina 
capillary tube having pervious sides. Any pressure brought to bear on the column of 
liquid contained in the porous duct of a plant, must quickly cause the expulsion of a 
contained air-bubble through the minute openings in the coats of the duct. The greater 
molecular mobility of gases than liquids, implies that air will pass out far more readily 
than sap. Whilst, therefore, a slight tension on the column of sap will cause it to part 
and the air to enter, a slight pressure upon it will force out the air and reunite the 
divided parts of the column. | 
To obtain data for an opinion on this vexed question, I have lately been experimenting 
on the absorption of dyes by plants. So far as I can learn, experiments of this kind have 
most, if not all of them, been made on stems, and, as it would seem from the results, 0t 
stems so far developed as to contain all their characteristie structures. The first exp“ 
riments I made myself were on such parts, and yielded evidence that served but little to 
elucidate matters. It was only after trying like experiments with leaves of differen 
ages and different characters, and with undeveloped axes, as well as with axes of special 
kinds, that comprehensible results were reached; and it then became manifest that * 
appearances presented by ordinary stems when thus tested, are in a great degree m 
leadi Let me briefly indicate the differences. 
Ifan adult shoot of a tree or shrub be cut off, and have its lower end placed in an alumed 
decoction of logwood or a dilute solution of magenta *, the dye will, in the course of a ft 
hours, ascend to a distance varying according to the rate of evaporation from the leave 
à * These two dyes have affinities for different components of the tissues, and may be advantageously used 
Pia iens Magenta is rapidly taken up by woody matter and other secondary deposits ; while logw ood co 
cell-membranes, and takes but reluctantly to the substances seized by magenta. By trying both ud 
same structure, we may guard ourselves against any error arising from selective combination. 
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