THE ARIZONA ASH (FRAXINUS ARIZONICA) 



Growing on the margin of a sandy streamway near Tucson, this Ash 

 is one of the valuable shade trees of the region to which it is native 



THE AUTOGRAPH OF AN ASH TREE 



D. T. MacDOUGAL 



Director of the Department of Botanical Research, Carnegie Institution of Washington 



Measuring the Growth of Trees — A New and Ingenious Method 

 of Exactly Recording Reactions and Seasonal Development 



Editors' Note: Formerly Assistant Director of the New York Botanical'Garden, Dr. MacDougal came well equipped to his present work of directing botanical 

 research, and the Desert Laboratory of the Carnegie Institution at Tucson, Arizona, offers abundant opportunity for direct experiment and study of many forms of plant 

 life. This authentic record of an Ash tree's growth has considerable scientific value and anything that throws fresh light on the fascinating and mysterious movements 

 of the plant world is fraught with significance for the gardener. Preceding articles of the series may be found in The Garden Magazine for February and June (1922). 



•HE daily variations in the ascent of sap and in rate of 

 growth of a tree are highly suggestive of pulsations, 

 and the seasonal changes of the activities of all plants 

 when viewed in the open give the impression of a 

 rhythm or alternating periods of rest and action determined by 

 or inherent in the organism. When we couple a plant to an in- 

 strument designed to register its changes, we may obtain a re- 

 cord from which it is apparent that it is highly responsive to the 

 conditions under which it lives. Growth begins when tempera- 

 ture, moisture and food-supply are adequate. Quickening, 



depression and stoppage result from changes in these factors. 

 A glimpse at some of the arrangements of cells in a tree and an 

 analysis of its autographic record for a season or two will put 

 the reader in possession of facts supporting these conclusions. 



The trunk of a tree of the type with which we are most fami- 

 liar begins as a thin, tapering, cylindrical shoot in the seedling, 

 and by the addition of thin conical shells or layers of wood each 

 year builds up the shafts which may reach majestic proportions 

 in some species. The young stem at first shows a central pith 

 soon becoming enclosed in a shell of woody cells, external to 



3'3 



