502 Thoday. — On the Organization of Growth and 
gives rise to typical secondary wood with vessels, fibres, and medullary 
rays. By this time the tangential growth has ceased, the primary medullary 
rays have been bridged by cambium, and a uniform zone of wood is formed 
by radial activity of the continuous cambium, as in a typical woody stem, 
until the growth of the plant as a whole comes to an end (Plate XVII, 
Fig- 3 )- 
From a morphological point of view this final resemblance to a woody 
stem cannot be regarded as a peculiarity of the basal region. What 
differentiates the latter from the upper part is the opportunity to reach this 
ontogenetic phase, an opportunity which is denied to the upper part by 
those physiological qualities of the plant as a whole which find expression 
in its limited life. 
Just as the secondary growth in thickness of the upper part of the stem 
follows, so far as it goes, the same lines as that of the basal internodes, so 
also the primary growth of the latter and the development of the trace 
bundles of the lowest leaves take place in the same way as has been 
described for the upper part of the shoot . 1 
Thus throughout the shoot the same mode of organization is revealed. 
The key-note is plastic adaptability. The mode of growth of the primary 
bundles is equally suitable for the production of the small bundles of the 
young plant, when food supplies are limited, and of the large bundles of the 
full-grown plant, with its root system well established and its resources 
fully mobilized. The mode of secondary growth in thickness is especially 
suited to the stem of a large annual plant, as it secures the necessary rigidity 
with great economy, and because the growth of the lower parts is so 
intimately correlated with, and responsive to, the course of development 
above. 
This correlation is further illustrated by the contrast between vigorous 
plants and small plants grown with their roots confined. A more extreme 
contrast is afforded by the meagre plants grown in sawdust and not trans- 
planted. The mature plants are little more than a foot high, bear small 
few-rayed capitula, and have stems which even in the lower internodes are 
hardly thicker than those of the young plant. As Text-fig. io illustrates, 
there is little secondary growth even in the lowest internodes ; additional 
rigidity is secured by the lignification of the perimedullary parenchyma. It 
is also of some interest that the decussate arrangement of the leaves persists 
till just below the inflorescence, five or six pairs being formed. 
This example shows the effect of mere starvation on the structure of 
a species which under favourable conditions shows a considerable amount of 
1 The only apparent exception is the growth of the cotyledonary traces ; but Chauveaud’s 
seedling studies (Ann. Sci. Nat., Bot., 9® ser., xiii, 1911, and other papers quoted therein) suggest 
that ‘ xylem superpose ’ is cambial in origin. Cf. also Lenoir, ibid., 1921. 
