Problem of Xeromorphy in Marsh Plants. 839 
comparatively slight ; the leaves appearing to be nearly as hairy as those 
grown in the open. The hairs, however, were distinctly shorter. The greatest 
diminution of hairiness was induced when low light-intensity was super added 
to stillness and humidity of the air. Thus ( b ) flowering shoots grown in weak 
light under glass shades still produced more or less hairy leaves, but the 
hairs were straight and very short. The lower cauline leaves appeared to 
be more easily influenced than the upper. 
A curious inhibition of hairs occurred on plants which had been 
protected from cold, &c., during the winter. A number of plants were kept 
indoors continuously for some years, in the Cool Fern House and the 
Cool Pit at Cambridge, and the Laboratories at Aberystwyth. In all 
cases the rhizomes of these plants remained horizontal, and each year 
produced a crop of radical leaves of the glabrous spring or autumn type. 1 In 
a few instances patches of hairs occurred on a leaf, but for the most part 
the leaves were entirely glabrous. On the other hand, plants brought 
indoors towards the close of the winter produced normal erect flowering 
shoots with hairy leaves. 
Another experiment which resulted in decreased hair formation is 
recorded on p. 847. 
Spiraea Ulmaria , var. denudata , Boenn., like the type form, commences 
in spring-time by forming glabrous leaves. These are followed by increas- 
ingly hairy leaves as the erect flowering stem elongates. But here the 
hairiness begins somewhat later, and is chiefly confined to the larger veins 
of the leaves ; the general pubescence, so characteristic of the type, being 
absent. In nature I have found many transitional forms intermediate 
between S. Ulmaria and its variety denudata . 2 Thus individuals are some- 
times found with an unusual number of glabrous radical leaves. In others, 
one, two, or even more of the cauline leaves may also be glabrous. Finally, 
plants occur in which only the leaves at the extreme top of the flowering 
shoot are markedly hairy. But in all cases, whether in the type, or its 
var. denudata , or in any of the numerous intermediate forms, the lower 
leaves are glabrous. On the other hand, the cauline leaves are (in varying 
degrees) increasingly hairy as the flowering shoots are ascended. 
inner surface. The method used was based on the following : If a solution of potassium iodide be 
mixed with dilute sulphuric acid, and exposed to light in the presence of oxygen, the iodine liberated 
is a measure of the total light received during the time of exposure (see Manchester (’ 93 ), p. 88 
et seq.). This method, to which Professor Weiss called my attention, has the advantage of recording 
the total light received , and not merely the intensity at a given moment. Like other photometric 
methods, it has the disadvantage of recording only the chemically active rays. For a discussion 
of the utility of such methods for botanical researches, see Riibel (’ 08 ), pp. 5-7. 
1 Gates (TO), p. 677, similarly found that the internodes of Oenotheras grown in a tropical 
greenhouse did not elongate, the plants thus remaining (for the entire time of the experiment, i. e. 
twenty months) in the rosette condition. Cf. also the experiments of Klebs (TO), p. 552 et seq., 
on Sempervivum. 
5 Trow (’ll), p. 58, also states that S. denudata occurs in Glamorganshire, together with many 
intermediate forms connecting it with the type, 
