834 Yapp. — Spiraea Ulmaria , Z., arid its Bearing on the 
§ 9. The Effect of Environment on the Production 
of Hairs and Palisade Cells. 
(a) Historical. Many of the older observers noticed that plants not 
infrequently varied in hairiness according to habitat. Thus Linnaeus 1 
mentioned Plantago Coronopus as being ‘ in humido foliis glabris integris, 
in sicco hirsutis dentatis *. 1 De Candolle 2 said that lymphatic (i. e. non- 
glandular) hairs ‘ are in general rare in plants which grow in the shade, or in 
rich and moist places * ; while ‘ they are more abundant, generally, in those 
which grow in warm and dry places, much exposed to the sun \ Kraus 3 
instanced potato shoots as being less hairy or even glabrous when grown in 
very moist air than in dry air. 
During the last thirty years a considerable literature has arisen, dealing 
with the influence of external factors, not only on the development of 
epidermal structures such as hairs, stomata, &c., but also on the differentia- 
tion of the mesophyll, and especially of the palisade tissue. One of the 
earlier works of this period was the classic paper by Stahl, 4 published 
in 1883, in which the action of light is regarded as the factor determining the 
structure of c sun * and ‘ shade ’ leaves. 
In the same year Mer 5 found that light and also dry air favour the 
production of palisade tissue, while shade and humidity produce the reverse 
effect. The same is true also of hairs on aerial stems. 6 A well-watered soil 
hindered the formation of both stem- and root-hairs, while both were 
produced if the soil were drier. 
Vesque and Viet 7 proved that light, accompanied by more or less dry 
air, increases the development of palisade tissue, and the number and length 
of hairs on leaves. In a later paper, Vesque 8 says that the result of a large 
number of cultural experiments shows that a marked development of pali- 
sade tissue can always be referred to the action of transpiration. 
Kohl 9 stated that changes of relative humidity in the atmosphere 
surrounding growing organs are capable of inducing the formation or 
suppression (as the case may be) of palisade cells. Again, he found 10 that 
in the case of hairy species of Compositae or Labiatae, not only the number 
of hairs, but also their length, is determined to a great extent by the amount 
of transpiration from the organs themselves. The less the transpiration, the 
fewer and shorter the hairs. 
Dufour 11 found that the dimensions of leaves developed in light of high 
intensity are greater in all respects (and the palisade tissue is more pro- 
nounced) than those of leaves grown in shade. 
1 Linnaeus (’63), p. 219. 
4 Stahl (’83). 
7 Vesque and Viet (’81), p. 176. 
10 Kohl, 1. c., p. 103. 
2 De Candolle (’41), p. 98. 
5 Mer (’83), p. 112. 
8 Vesque (’83), p. 489. 
11 Dufour (’87), pp. 406-8. 
3 Kraus (’-7 6), p. 153. 
6 Mer, 1. c., p. 1 1 7. 
0 Kohl (’86), p. 93. 
