86 Growth 



products of the cambium, through the permanent record they leave in the 

 tree, are of importance in the study of past climatic changes and in 

 the dating of ancient timbers, and have been actively studied especially 

 by Schulman ( 1956) and others in the Laboratory of Tree Ring Research. 

 The course of cambial activity differs among various plants. In herba- 

 ceous ones it is associated with vegetative growth and generally ceases at 

 flowering (Wilton and Roberts, 1936). In most woody plants, growth of 

 the new shoots in length is complete or nearly so before there is much 

 cambial activity. The age of the tree may make a difference in woody 

 plants, for Messeri (1948) observed that in old trees cambial divisions 

 began a month earlier in the twigs than in the main stem whereas in 

 young ones they started simultaneously throughout. In most conifers and 

 in ring-porous angiosperms, cambial growth begins at about the same time 

 throughout the stem (Wareing, 1951). In diffuse-porous angiosperms, 

 however, division starts just below the buds at about the time they open 

 and proceeds downward into the branches and then the trunk (Cocker- 

 ham, 1930; Priestley, Scott, and Malins, 1933). This is apparently related 

 to the production of auxin in the buds, for there is a close relation be- 

 tween the appearance of auxin there and the onset of cambial activity 

 (Avery and others, 1937b). 



The relation of auxin to cambial growth has also been studied by Brown 

 and Cormack (1937), Soding (1940), Kiinning (1950), and others. 

 Chowdhury and Tandan (1950), working with both evergreen and de- 

 ciduous trees in India, report that buds burst in February or March and 

 that growth in length continues until May. Not until the new leaves are 

 fully expanded and length growth has ceased does cambial activity begin. 

 It starts at the tip of the last year's shoot and proceeds down the tree and 

 up into the new shoot. Growth in length begins again in the summer 

 and is accompanied by cambial activity until both cease in the fall. The 

 authors suggest that there are two types of substances operating here, 

 one concerned with apical growth and one with cambial. Aspects of cam- 

 bium physiology were discussed by Priestley in a series of papers ( 1930 

 and others). 



A continuous cambium, laying down a solid ring of vascular tissue, is 

 found in all woody gymnosperms and angiosperms and in many herba- 

 ceous forms. In other herbaceous stems the cambium ring is discontinu- 

 ous and produces distinct vascular bundles. These may be separated by 

 undifferentiated fundamental tissue. Across these gaps and connecting the 

 cambium of one bundle with that of the next there is often an inter- 

 fascicular cambium, probably a vestigial structure persisting from the 

 time when the cylinder was continuous and the stem was woody. In many 

 cases it consists of a row of cells with only a few tangential divisions. 

 Under suitable conditions, as in the base of a stout herbaceous stem, the 



