492 Thoday. — On the Organization of Growth and 
expect . 1 There are, however, other correlations between hypocotyl and 
root which require further investigation. 
Course of bundles. As the arrangement of the leaves is at first 
opposite and decussate, but later alternate, the course of the leaf-trace 
bundles shows a corresponding change. In a plant growing under favour- 
able conditions the change of phyllotaxy begins after about four pairs of 
opposite leaves have appeared. The trace bundles of the third pair pass 
down through three internodes and then usually fork, but sometimes deviate 
without forking, so avoiding the incoming leaf- traces of the first pair. The 
median bundles of the first pair similarly fork past the lateral cotyledonary 
trace bundles. If the leaves at alternate nodes were exactly superposed, 
the bundles would typically pass through two internodes, then fork, each 
branch being joined by a similar bundle from the node above. This ideal 
arrangement is only approximated to even at the base of a vigorous plant . 2 
In a particular stem, in which the bundles were traced in detail, the fourth 
pair of leaves already showed a small displacement, the next two leaves 
deviated still more and were separated by a short internode. The lateral 
displacement of the leaves involves the leaf-traces also and the bundles 
tend more and more to miss incoming leaf-trace bundles at lower nodes 
and so to traverse a larger number of internodes independently. 
The transition to an approximately two-fifths phyllotaxy is rapid. 
When this arrangement is fully established the bundles can be followed for 
about five internodes, sometimes even farther, before they are joined by 
one or more minor bundles, after which they traverse one or two more 
internodes, sometimes fork to avoid incoming bundles, and, diminishing in 
size, finally attach themselves to neighbouring bundles. Sometimes they 
end blindly as primary bundles ; but secondary continuations link them to 
neighbouring strands. 
There is nothing stereotyped about this disposition of the bundles. 
The facts can best be understood as a solution of the problem of accom- 
modating the traces of an increasingly rapid succession of leaves. The 
space available diminishes downwards : firstly, by reason of the entrance of 
other traces at each node ; secondly, because towards the base of the plant 
all the primary tissues are on a smaller scale. Trace bundles from the 
upper leaves are therefore crowded into narrower spaces and ultimately 
unite with one another. It is these synthetic traces that grow tangentially 
and that form the massive wood-sectors so conspicuous at the base of the 
stem. They always communicate directly with the upper, still expanding 
part of the shoot. Moreover, their growth is correlated with that of the 
1 It may be thought that these accommodations have been emphasized unduly, but Jeffrey and 
others who have started with a phylogenetic point of view seem to have entirely missed them. See 
below, pp. 503-5, for further discussion. 
2 But a close approximation to it is found in starved dwarf plants, where it persists through 
several internodes. Cf. p. 502. 
