10 



MACDOUGAL— GROWTH IN TREES. 



collapse. Something of this sort takes place in many trees which 

 have been kept under observation. The conduit in this case however 

 is not a simple pipe or a set of pipes, but is made up of vessels 

 through which water may pass under capillary conditions, and en- 

 closed box-like tracheids which may be only partially filled with 

 water. When water is withdrawn from such a system faster than 

 it is taken in the resulting changes in form and size are complex in 

 character, but are expressed by the well-defined daily equalizing 

 variations which are of a characteristic type for. each kind of tree 

 (Fig. 5)- 



JULY 5-12-1920 



6AM 12 6PM I? 6AM.g 6PM IZ. SAM 12. 6PM 12. BAM 12, 6PM,.IZ 6AM 12.6PM \1 BAM IZ 6PM IZ BAM 12 6PM 12 6AM IZ 



Fig. S. Dendrographic record of Arizona ash near the end of the grow- 

 ing season. The daily equalizing variation is large, but the diameter shows 

 an increase on each successive day. Variations X lo on a scale of lo mm. 

 intervals. 



The greatest daily equalizing variations were shown by Fraxinus, 

 Pinus, Picea, Pseudotsuga and Juglans, and lesser variations were 

 displayed by Populus, Platanus, Fagus, Quercus and Citrus. No 

 available facts furnish the basis of an adequate explanation of such 

 differences. 



Estimates of the range of daily equalizing variations in a Mon- 

 terey pine taken from bearings on a thin layer of cork external to 

 the bast of a trunk which had ceased to grow for the season shows 

 that the diameter might vary one part in 1,750. That a large share 

 of this variation is due to changes in the hydration of the living 

 cells is proved by the fact that when bearings are taken on the woody 

 cylinder of the trunk internal to the growing layer the variation 

 drops to one part in 8,750 of the diameter. The actual change in 

 volume in the first instance calculated on the basis of a conical trunk 

 18 meters high and 35 cm. in diameter at the base would amount to 



