KNUDSON: CAMBIUM DEVELOPMENT IN AMERICAN LARCH 275 



the leaves being 1/16 of an inch in length. On the younger wood, 

 the buds were less advanced. This was more marked in the I- and 

 2 -year old wood. This slower development toward the terminal 

 shoot and apex of the branch held true also for the catkins. The 

 same condition was noted also in several larches which in Sep- 

 tember produced a new growth of leaves, the result of a drouth, 

 followed by favorable conditions. This earlier development of 

 leaves on the older wood is significant in the light of the subsequent 

 facts concerning the inception of cambial activity. 



Cambium in resting condition. According to Sanio and other 

 investigators the cambium proper consists of a tissue but one cell 

 in thickness, which cells by division produce a row of xylem mother 

 and a row of phloem mother cells. Each of these rows divides and 

 gives rise respectively to two rows of potential xylem cells and 

 two rows of potential phloem cells. Except by careful cytological 

 study the row of true cambium cells cannot be distinguished from 

 the neighboring cells. The term cambium has been, therefore, 

 generally applied to that tissue which lies between the visibly 

 differentiating phloem and xylem. The cambium tissue is com- 

 posed of a number of rows of cells, which cells are characterized by 

 their thin walls, dense protoplasmic content, and, viewed in 

 cross section, rectangular shape. In trees in the resting condition 

 it w T ould be reasonably assumed that the true cambium comprises 

 the first row of cells just without the xylem. The cells bordering 

 this row on the outside would then be considered as phloem. 

 As a matter of fact, however, sections made from cuttings obtained 

 from the trunk of larch on November 13 exhibit just outside of the 

 xylem a distinct tissue 34 M in diameter, consisting of five or six 

 rows of cells in thickness. The cells of the outer five rows are not 

 visibly distinguished from cells of the inner row, but are distin- 

 guished from the adjacent phloem cells by their size and proto- 

 plasmic content. See FIG. I and 3. Because of the similarity 

 of all of these cells I have considered the six rows as comprising 

 the cambium tissue, the term cambium being employed in its 

 generally applied sense. 



Inception of cambial activity and development of phloem. The 

 material collected on April 19 showed that cambial activity had 

 begun. The layer of six cambium cells had increased in diameter. 



