174 The Living Plant 



ture of molasses and water, that which rises in the ducts is prac- 

 tically pure water. This difference, obviously, is correlated with 

 a difference of sti-ucture, viz., in the plant the water has to pass 

 through intermediate cells, which are wanting in the osmo- 

 scope. We have already learned why it is that the sugar does 

 not pass with the water into the ducts (the protoplasm stops 

 it), and our problem resolves itself into this, — how is it that 

 the cortical cells send water into the ducts when all of the con- 

 ditions seem rather to invite the absorption of water from them, 

 exactly as the hairs absorb it from the soil? This question, 

 I am sorry to say, I cannot yet answer, for it remains one of the 

 unsolved problems of plant physiology, though one of the most 

 inviting of them all. It is true, some physiological books at- 

 tempt to explain it, but in all cases, so far as I have observed, 

 either their physics is bad, or else their explanations are worded 

 in a manner more lethal than logical. I suspect the explana- 

 tion will ultimately be found in some ordinary physical or chem- 

 ical processes working under control of some still unknown prop- 

 erty of the protoplasm. 



In the three or four paragraphs which follow I purpose to 

 explain how it is that substances in solution can pass through 

 imperforate membranes, and what are the forces which drive 

 them. The subject involves a consideration of molecules, and 

 things of that sort, and will require hard work from the con- 

 structive imagination. So the reader is given fair warning and 

 may skip if he pleases, though I beg to remind him that this 

 book makes appeal to his reason, and is an attempt to help him 

 to share the spartan pleasures of understanding. 



The most striking feature of osmotic absorption consists in 

 the remarkable rise of a large body of liquid against gravitation 

 without the operation of any visible forces whatsoever. Yet 

 forces there must be, if not visible, then invisible; and accord- 

 ingly we turn for the sources of the power deep within the con- 

 stitution of the bodies themselves. As everybody knows, mem- 



