8 38 yapp. — Spiraea Ulmaria, L., and its Bearing on the 
(c) Experiments on Spiraea , &c. In the first instance I was easily 
able to confirm the researches of Vesque and Viet, Kohl, and others, 1 
on the effect of humidity on hair production in certain plants. E. g. in 
Epilobium hirsutum and Mentha aquatic a, the formation of hairs was com- 
pletely inhibited by growing the plants in a damp atmosphere ; the converse 
being true of a dry atmosphere. The case of .S'. Ulmaria, however, proved 
to be somewhat different. A large number of plants of this species were 
grown, some at the Cambridge Botanic Garden (through the kindness 
of Mr. R. I. Lynch), and others at Aberystwyth. A wide range of habitats 
was secured by varying the following factors: (r) soil, (2) soil-moisture, 
(3) humidity of air, (4) light-intensity, and (5) degree of exposure or 
shelter. 2 
The experiments extended over a period of four years, so that while 
the same individual plants were used, several generations of shoots were 
observed. 
On the whole, the effect of varying habitats on the hairiness of Spiraea 
Ulmaria was less than had been expected, the character being apparently 
one of a certain degree of fixity. Differences of soil, soil-moisture, light- 
intensity, and exposure produced no effect whatever on the normal perio- 
dicity of hair formation. Under no conditions could the normally glabrous 
spring or autumn leaves be induced to become hairy, 3 while in all cases the 
upper leaves on the flowering shoots produced hairs. 
But though external conditions had comparatively little controlling 
effect on the actual formation of hairs, in certain instances they exercised 
considerable influence on the frequency, and especially on the length of the 
hairs. This was the case with shoots grown under glass shades, i. e. in still, 
humid air. When [a) these were exposed to full sunlight, 4 the effect was 
1 Vesque and Viet (’81), Kohl (’86), &c. 
2 At Cambridge the plants were potted in either ( a ) pure peat, (b) loam, or (c) a mixture or 
peat, loam, and sand. The pots were then divided into two sets. One set was placed in a fully 
insolated part of the garden, the other in a shady spot, never reached by the direct rays of the sun. 
Soil-moisture was varied by submerging (partly or completely) certain pots in water. Others had 
good drainage, but were copiously watered, while a third set were only watered at infrequent intervals. 
A final differentiation of habitat consisted in placing glass shades over some of the drained and also 
undrained pots. The aerial shoots were thus developed in a still, humid atmosphere, under varying 
conditions of light, soil-moisture, &c. 
At Aberystwyth plants were grown in various situations in a garden, some in deep shade, others 
in the open, &c. Others were kept in the laboratories, and also on the roof of the College, which 
is situated close to the sea, and exposed to great gales of wind. 
3 Cf. Vesque (’84), p. 21, who says that he had never succeeded in inducing a single hair to 
form on a really glabrous plant : though theoretically he thought it should be possible, as ‘ les 
plantes glabres ont du pr^ceder historiquement ’. 
4 As determined by experiment : — 
(1) The shade plants ( b ) were grown under light-intensity = i-o 
(2) „ sun „ (a) „ „ „ „ = 4*5 
(3) In the free air outside the glass shade («) „ = 4-9 
Thus, a little light is cut off by the glass shade, or by the moisture which condenses on its 
