LABORATORY FOR PLANT PHYSIOLOGY.^ 



D. T. MacDougal, Director. 



The activities of the members of the staff have been devoted to three 

 main subjects — photosynthesis and metaboHsm, permeabihty and growth, 

 and to certain ecological relations of plants. Progress has been made in the 

 study of some of the special problems as described in the following paragraphs. 



GROWTH AND PERMEABILITY. 

 Dendrographic Records of Growth in Trees, by D. T. MacDougal. 



The series of observations on the course of growth in the trunks of trees, 

 which was begun with the first design of the dendrograph in 1918, has been 

 continued and records amounting to 20 seasons have been secured, making 

 a total available of about 90 seasons, the greatest number being of the Mon- 

 terey pine, one tree of which has furnished a continuous record for 5 years. 



The general behavior of the Monterey pine now being fairly apprehended, 

 many series of experiments have been begun which may yield evidence upon 

 the mechanism of transport of material and of physical action in growth. 

 Girdling, defoHation, decapitation, injections of material, and increase and 

 decrease of pressures have all been used with some decisive results which will 

 be presented in forthcoming publications. 



The dendrographic results in connection with the measurements of trunks 

 made by Dr. Shreve as described in a separate paragraph establish the fact 

 that the thickness of the layer of wood formed in the Monterey pine does not 

 correspond directly with the rainfall of the season of its formation, of the 

 previous season, or of any known phase of such precipitation. The amount 

 of wood formation is therefore not a direct record of rainfall in this tree and 

 should not be assumed to be in any tree until it has been proved to be such 

 an index. Extended study of cross-sections of the trunks of the yellow pine 

 supports the conclusion that in this tree the amount of wood formed in any 

 year does show a fairly close correspondence to the amount of precipitation. 



Formation of wood, and growth in general, is the result of the integrated 

 or correlated action of several factors, including soil moisture, temperature, 

 hght, and food-supply. In some species water may be the dominant agency, 

 and high precipitation in the habitats of these species may be connected 

 with optimum intensities of the other contributory factors. On the other 

 hand, it is doubtless true that in some regions the season with the most rain- 

 fall may have unfavorable conditions in other respects. Something of this 

 kind seems to be the case with the Monterey pine. Six seasons' records of 

 the redwood (Sequoia sempervirens) have been made, and it seems advisable 

 to extend the observations under experimental conditions to ascertain the 

 influence of the amount of precipitation as well as other features upon the 

 amount of growth. The course and localization of growth in a girdled tree 

 have been followed from the time of operation until its death, 14 months later. 



The woody cylinder of a living tree has been found to show a daily variation 

 in diameter amounting to 1 part in 1,550 in March, which decreased to 1 



' Reorganized Department of Botanical Research, approved by Executive Committee May 

 18, 192.3. Situated at Tucson, Arizona, and Carmel, California. 



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