STRUCTURAL VARIATIONS IN HOMOLOGOUS CELLS 155 



Provided that such material is collected early enough in the growing 

 season, then it is found that all the cells are of this kind, and have densely 

 cytoplasmic contents without the obvious vacuoles which will appear 

 later in derived cells. This layer then constitutes cells newly cut off from 

 the cambium, the tenuous living layer of the stem, which produces during 

 each growing season the phloem on the outside and the xylem, or wood, 

 on the inside, and is therefore responsible for the growth in thickness of 

 the stem year by year. Towards the end of each season this cambium 

 ceases its activity and stiffens in consistency, presumably due largely to a 

 lowering of the water content, and from then on until the onset of the 

 next growing season the bark, cambium and xylem adhere together 

 very firmly. During spring, however, cambial activity is again renewed, 

 so that year by year this tenuous living layer lays down a new cylinder 

 of wood surrounding the old, and this continues throughout the life of 

 the tree. These yearly increments of wood cause the appearance on the 

 cross-section of a trunk of the so-called annual rings. 



The production of wood tracheids is in some ways a most re- 

 markable business. The cells of the cambium are quite uniform in 

 shape, taking the form of long, thin cells, much narrower in the radial 

 direction than in the tangential, and with six longitudinal faces much as 

 described and figured in previous pages (Chapter II) (Fig. 5A{a)). These 

 cells are growing, increasing largely in radial dimensions (but, as we shall 

 see, also in length) by the continuous production of new protoplasm 

 until, when some undefined size has been attained, they divide into two 

 cells. This division is pecuUar, however, in that the division wall does 

 not lie transversely, cutting the cell into two of the same width but one- 

 half the length. Instead, it lies longitudinally, cutting the cell into two 

 of the same length but of only one-half the width. Further, this divid- 

 ing wall always lies tangentially to the stem so that the two daughter 

 cells he along a radius and never along a tangent to the stem. The 

 occasional transverse divisions (or pseudo-transverse since the dividing 

 wall is never really horizontal) are too few to be of account here; and in 

 any case such a dividing wall rapidly swings round to become a new 

 radial wall, by a mechanism which is not understood. The divisions 

 with which we are most concerned, therefore, are longitudinal tangential. 

 Now when such a division occurs on the inner face of the cambial 

 cyhnder, the two daughter cells (Fig. 5A{b)), commonly behave in a 

 dissimilar manner. We shall consider here only the case in which the 

 outer of them remains a cambial cell. This continues to grow as before 

 until a further division ensues. The one facing the wood, however, 

 increases in radial dimensions to a much greater extent; during the 



