PHENOMENA OF PLANT-LIFE. 29 



That living cellulose has a peculiar and very powerful 

 affinity for water is evident from the experiments of De Vries, 

 who discovered that when a shoot of an herbaceous plant with 

 large leaves is cut, and the fresh surface allowed to come for 

 a short time into contact with the air, it loses much of its 

 absorbing power and the leaves wilt. If, however, the 

 section be made under water, so that the living tissue is 

 not exposed to the air, its power of imbibition remains 

 unimpaired, and the leaves do not wilt. 



It appears, therefore, that much of the crude sap passes 

 through the membranes of the sap-wood or woo^y fibre or 

 cellular tissue of plants in an apparently solid form, combined 

 with the cellulose, just as the water in dry slacked lime, or a 

 plaster cast is in a solid form. In all these cases it may be 

 obtained as a liquid by distillation at a temperature of 212 

 Fahrenheit. The cause of the motion seems to be the removal 

 of the water from the tissue at some point by exhalation, by 

 chemical combination or by assimilation. Whenever any 

 portion of the living cellulose has an insufficient amount of 

 water to saturate its affinity, it imbibes an additional quantity 

 and this process is continued from cell to cell downward, or 

 backward to the roots and the earth. 



The conducting power of the cellulose of sap-wood is very 

 remarkable, as is seen in the fact that whenever a limb of an 

 apple or peach tree breaks down under its burden of fruit, it 

 very rarely wilts or fails to ripen its crop. Those who have 

 compared the area of a section of the trunk of a large tree 

 with the area of a section of its branches at any point above, 

 must have noticed that the relative amount of sap-wood 

 rapidly increases as we ascend toward the top, the young 

 twigs and branches containing no other wood. 



An elm in Amherst, famous for the beautiful symmetry of 

 its form and known as the Ay res elm, was carefully measured 

 by Prof. Graves and the senior class. The area of the sec- 

 tions of the branches twenty feet from the ground was more 

 than twice as great as the area of a section of the trunk four 

 feet from the earth, and the proportion of sap-wood was of 

 course much greater. 



An interesting experiment was undertaken in the Durfee 

 Plant-house to determine how small a proportion of sap-wood 



