276 KNUDSON: CAMBIUM DEVELOPMENT IN AMERICAN LARCH 



The outer cells of this tissue were losing their rectangular shape 

 and assuming more nearly that of a square, as viewed in a cross 

 section. See FIG. I, 2, and 3. In studying the slides made from 

 material collected on April 19, it was found that in the 5th, 6th, 

 and 7th cuttings of tree A the cambium had developed to a greater 

 extent than in the other cuttings. Not only had the cambium 

 increased in diameter, but seemingly new cells had been formed. 

 This increased development near the middle was maintained until 

 May 25. The average increase in number of phloem cells by May 

 25 was only 1.8, but the diameter increase of the cambium and 

 phloem was nearly 100 per cent. Up to this time no xylem what- 

 soever had been developed. It appears therefore that the earliest 

 growth consists in an enlargement of the cambium tissue with the 

 gradual transformation of the peripheral cells into phloem tissue. 

 The old phloem cells adjacent at this time are becoming compressed 

 due to the pressure brought about by the transformation of the 

 cambial cells. Compare FIG. 1,2, and 3. 



In TABLE I are given the figures obtained by the measurement 

 of the diameter of phloem and cambium tissues in trees A and C. 

 The figures for the diameter of the cambium tissue during the 

 resting period (cutting made November 13) are given for com- 

 parison. The figures included under the dates April 19 to May 

 25 inclusive refer to tree A. From June 3 to July 6 the figures 

 refer to tree C. 



As indicated previously, the six-celled layer adjacent to the 

 xylem is considered the cambium. Although transformation of 

 the peripheral cells had occurred, it is difficult to state which cells 

 are cambium and which cells are phloem. Consequently the six 

 rows, despite the transformation, I have considered as cambium. 

 Any cells in excess of the six rows, which lie within the old com- 

 pressed phloem cells, I have considered as new phloem. After 

 May 25, when xylem and phloem were both developing rapidly, 

 the cambium tissue was still considered as a tissue of six rows of 

 cells. It was difficult to select always the six most uniform rows, 

 but in general the error was slight and at most of little consequence. 



From an examination of the table it may be seen that up to 

 May 25 the middle regions show the greatest growth. From April 

 19 to May 25 the increase in phloem was gradual, but from May 25 



