i yo The Living Plant 



sponge," for the tip of the root, which was supposed to soak up 

 water and pass it on to the ducts. Later it was found that the 

 water enters chiefly through the hairs. But in these the condi- 

 tions for capillarity are absent; for capillarity requires openings, 

 and the hair walls contain none that even the most powerful mi- 

 croscopes can detect. The problem is, therefore, to explain an 

 absorption of water through membranes that are imperforate, or 

 solid, and through protoplasm-lined and sugar-holding cells into 

 protoplasm-less and sugar-less ducts. But the very mention of 

 absorption into sugar solutions through imperforate membranes 

 immediately suggests a direction for our further inquiry, since it 

 recalls a mode of absorption very well known in physics, and as- 

 sociated with those very conditions, viz., Osmosis. 



So important is this subject of osmosis to an understanding not 

 only of absorption of water by plants, but of many other notable 

 phenomena as well, that the reader ought really to make its more 

 intimate personal acquaintance. This he can do by aid of the 

 following experiment, which is one familiar to all workers with 

 plant physiology. Over the end of a large glass tube is tied 

 firmly, by means of waxed thread, a piece of soaked parchment, 

 (preferably a cylindrical parchment cup made for the purpose) 

 which is a physical equivalent of the wall of the root hair; into the 

 tube is poured a solution of sugar, for which molasses, a solution 

 ready made and conveniently colored, is excellent ; then the tube 

 is supported with the parchment in pure water, the whole arrange- 

 ment being much as displayed in the accompanying picture 

 (figure 56). A surprising result always follows, for, without the 

 operation of any visible machinery or forces, and in a manner 

 which to me, despite long familiarity therewith, looks always 

 anomalous and even somewhat uncanny, the liquid rises steadily 

 though slowly in the tube, lifting its own considerable weight, 

 until within two or three days it has reached a height of two or 

 three feet or more. Indeed the process can be demonstrated 

 even more strikingly than this, for, if the parchment cup be 



