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 2. 
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 
