49 ° Thoday. — On the Organization of Growth and 
In the Sunflower stem, on the contrary, the pith increases greatly in 
diameter and the bundles become widely separated. Text-figs, i and 2 
represent sections through the third internode above the cotyledons of two 
plants of different age. Both are drawn to the same scale and the insetting 
of Text-fig. 2 in Text-fig. i serves to emphasize the point in question. 
The diameter of the pith has increased approximately threefold during the 
growth that has occurred between the two stages represented. Both plants 
were grown in the same plot, and at the earlier stage the diameters of their 
stems were of the same order of magnitude. 
In the older stage certain of the primary bundles are still readily 
distinguishable. Six of them (/, i, i and /', i\ /), which belonged to the 
pair of leaves at the node next above, are conspicuous by reason of their 
small radial extent — the fascicular cambium has not been very active. 
Two other bundles (II and II') are the median bundles belonging to the 
pair of leaves at the second node above. In these the cambium has been 
more active radially. But all these eight bundles and a few others are 
alike in their small width, which is little if at all greater than the width of 
the original primary bundle (Text-fig. 2). The width of the intervals 
between them, on the other hand, has increased enormously, and in these 
intervals broad wedges of secondary wood have been formed. 
In the enlarged pith, lines of strain are expressed in the arrangement, 
shape, and dimensions of the cells (Text-fig. 3), which clearly indicate that 
the pith has not itself been the active agent in the expansion, but has 
suffered enlargement more or less passively. Indeed, in still older stems it 
yields under the strain and becomes hollow (compare Plate XVII, Fig. 5). 
The active growth has been located in the cambial region and the expansive 
force exerted in the tangential direction. 1 In this tangential growth the 
cambium of the principal bundles has taken little or no part. 
Further information is obtained from a closer study of the older section. 
Text-fig. 3 illustrates a part of it on a larger scale. The radial lines in the 
wood mark the direction of rows of elements formed by individual cambium 
cells : their divergence in parts of the sectors between the principal 
bundles is clear evidence of tangential growth at these points (see also 
Plate XVII, Fig. 4). It is also evident, however, that the sectors have 
been augmented by the extension of cambial activity into the adjoining 
medullary rays, which have widened considerably to accommodate it. On 
either side of each of the principal bundles the rays have widened still 
further, and are now bridged by cambium. Here the cambium has not 
given rise to any secondary wood, and therefore is either recent or relatively 
1 Cf. Sachs, Textbook, p. 125 : ‘This increase in diameterof stems which accompanies, or 
even for a short time outlasts, the growth in length, is frequently occasioned mainly by the 
tangential extension of the outer layers of tissue, while that of the pith does not keep pace with it. 
The pith will then split and the stem become hollow.’ But Sachs appears to refer here rather 
to the cortical parenchyma. 
