TRANSACTIONS OF SECTION K. 711 
is visible to the eye in many instances where the two xylems are seen to run 
quite apart from one another for » great distance. 
Wound structures are usually said to be primitive; if so, they are very 
instructive, for we have seen that the bundles thus influenced assume a con- 
centric appearance and repeat the stages met with in the base of the petiole. 
We could then trace along either of these lines the history of the evolution of 
the foliar vascular tissue. Unfortunately the assumption is not yet proved 
as a general principle, and we are perhaps better inspired in explaining these 
abnormal structures as an attempt made by the plant to meet a physiological 
need. 
As far as evidence from traumatic structures is concerned, the problem 
stands where we left it. Our final conclusions are therefore :— 
The cfx. at the base is in great part a secondary growth, the cpx. is a 
primary structure; both are consequently independent morphologically. The 
two xylems overlap at their ends, are connected for a physiological function, 
and their reducéd extremities point to a time when possibly they ran parallel 
throughout their whole course. 
6. Variation of Structure and Colour of Flowers under Insolation. 
By Colonel H. E. Rawson, C.B., R.E. 
This paper was a continuation of one communicated to Section K in 1908, 
entitled ‘ Colour Changes in Flowers produced by Controlling Insolation.” The 
same method has been followed throughout of shading off with a perfectly 
opaque screen all direct rays of the sun for certain selected intervals of daylight, 
while admitting all the diffuse light possible. In the conservatories the condi- 
tions have been those of ordinary domestication, an uralite roof to the lantern, 
with the shelves and framing being sufficient, together with the aspect of the 
buildings, to modify the sunlight as required. In the open garden vertical 
screens, in combination with others at a height and not necessarily overhead, but 
capable of being raised or lowered, sliding upon a fixed bar, and set horizontally 
or at any desired angle, have been adopted. 
The garden nasturtium (Z'ropwolum majus) has continued to be used for the 
experiments, the history of each individual being carefully kept from the 
beginning in April 1905, but plants grown from English seed have been added 
for comparison with those from the seeds brought from South Africa. The same 
care has been taken as previously to keep all the conditions but those of ex- 
posure to direct sunlight the same, all the plants being looked after by one 
person and no fertilisers of any kind employed. 
The new mauve variety obtained in Pretoria by the method of screening, 
whose seed was brought to England in 1907 and came true, now flowers and 
seeds freely in any aspect and amongst any other varieties. Three plants left 
behind with a friend and not screened at the selected intervals gave less and 
ae seed each year until no seed matured, and in 1911 the variety became extinct 
there. 
Sterility.—When any material change has been made in the colouring- 
pigments of the petals the flowers always show a marked tendency to be sterile. 
In one case only two seeds were obtained from forty-three flowers. Nothing 
abnormal may appear in the gyneceum, but the ovary does not develop and soon 
atrophies. Seeds of the mauve variety are now frequently of a purple colour, 
and the syncarpous gyneceum may consist of as many as five coherent carpels. 
On the other hand, the gynzeceum is sometimes considerably modified, and in 
one case was apocarpous, four carpels being symmetrically arranged round a fifth 
in the centre, which pushed out a succession of ovaries until the whole was 
10 mm. long. This plant was one of three which under somewhat similar con- 
ditions grew a secondary flower directly out of the seed of the first and developed 
a second seed therefrom. One of the latter seeds put out a small stalked leaf 
from a seam at the top, the blade of which was apparently designed to protect 
the base of the pistil from the full sun to which it was being exposed. On 
observing this the whole seed was enclosed in a calico bag, and it was the only 
one of the three to mature. In all three cases additional whorls of leaves 
appeared in the gyneceum so as to surround and cover the reproductive organs, 
