DEPARTMENT OF BOTANICAL RESEARCH. 59 



cell-masses, show a relatively rapid rate of enlargement by increase 

 in length or in thickness. As maturity is approached, however, 

 material subject to the ordinary out-of-door condition displays a les- 

 sened growth at midday or under certain conditions indicative of the 

 fact that growth consists very largely of the imbibition of water and 

 that water is also being lost from the surfaces of the growing mem- 

 bers. So long as the amount of water taken up by the cell-colloids 

 of the organ is in excess of that transpired by its surface, an enlarge- 

 ment ensues. If, however, at any time of the day or under any con- 

 ditions the transpiration exceeds absorption, cessation of growth and 

 actual shrinkage are inevitable. Such occurrences of lessened volume 

 are illustrated by scores of records now on file. 



In addition to shrinkages being caused by a deficit of water or of 

 a loss exceeding the supply, there are also other changes, the physi- 

 cal nature of which is not obvious without an analysis of the con- 

 ditions existing within the tissues. Among these conditions is the 

 matter of acidity. It has been found that the colloids or cells of some 

 plants are capable of imbibing or holding less water at temperatures 

 of 25° to 30° C. than at 15° to 20° C. Consequently, when the temper- 

 ature of such plants rises an actual contraction ensues which is not to 

 be identified with any '^ optimum" or maximmn. 



Shrinkages or decrease in volume become more marked as maturity 

 is approached. As a consequence there exists a state of affairs in a 

 growing sunflower stem by which the extreme terminal portion will 

 be found in a condition of continuous enlargement, the internode 

 below will show daily shrinkages or cessation of growth in thickness 

 as measured by an auxograph arranged for that purpose, while the 

 internodes below will show even greater variations in volume. The 

 older parts of the stem consist very largely of mature tissues which 

 take up and give off water in a manner entirely reversible, and in 

 organs or members in which the embryonic tissue is reduced to a 

 minimum the widest range of reversible alterations occurs. 



The estimation of the balance between water-loss and water-supply 

 implied above has given a new and more important meaning to rela- 

 tive humidity as affecting growth. 



The Dendrograph: A New Instrument for recording Growth and Other Variations 

 in the Dimensions of Trees, hy D. T. MacDougal. 



The studies in growth which have received such a large share of 

 attention in these laboratories during the last few years have been 

 extended to include observations on a wide variety of material, in 

 order to test the validity of the general principles which are taking 

 form as a result of this work. Simple adaptations of the auxograph, 

 mentioned in previous reports and described in detail in various papers, 



