592 Hill. — Hairs on Stems , &c., of Tr op aeolum per egrinum, L. 
to produce a hairy or glabrous condition, though characters of hairiness 
or the reverse are known to be variable 1 among plants according to cir- 
cumstances. 
Vesque 2 discusses the development of hairs, and agrees with the 
theories of Kraus and Mer that the formation of hairs is due to the excess 
of nutritive matters and to the arrest in growth of the organs which carry 
them. He adds, ‘Des plantes differentes sont plus ou moins propres 
a developper des poils sous l’influence du milieu. Tandis que certaines 
especes deviennent velues au point d’etre m^connaissables, je n’ai jamais 
reussi a faire ddvelopper un seul poil sur une plante reellement glabre, sur 
le pois, par exemple.’ 
Vesque would possibly not consider Tr op aeolum per egrinum to be a 
truly glabrous plant, and it is unfortunate that his paper was not brought 
to my notice until it was too late to make similar experiments on peas to 
those made on the Canary-creeper. 
It is of interest to notice that the allied species, Tropaeolum majus, the 
common ‘ nasturtium * of gardens, is always hairy on the under surfaces of 
the leaves, and when grown under unfavourable conditions the starved plants 
tend to show an extension of the hairs down the short petioles. But when 
the plants receive favourable treatment the petioles become elongated and 
glabrous. The hairs, however, in T. majus , which are normal organs of the 
plant, differ from the straight unicellular hairs of T. per egrinum in being 
multicellular and curved. The hairs which arise from single epidermal 
cells may consist of from three to several cells, the terminal cell being 
usually the longest. 
The cause of hair production in T. peregrinum may perhaps be due to 
the excess of water supplied to the plant in its abnormal condition, and to 
the sudden arrest of growth owing to the removal of the large evaporating 
and growing surface represented by the laminae. It was not obvious that 
the hairs performed any marked function connected with transpiration, 
though it may well be that they represent the plant’s efforts to provide 
a substitute for the large transpiring surfaces which it has lost 
EXPLANATION OF PLATE LVI. 
A photograph of the upper part of a plant deprived of its laminae and showing a considerable 
development of epidermal hairs. Slightly enlarged. 
1 Yapp on Spiraea Ulmaria. Brit. Assoc., Portsmouth, 1911. 
2 Sur les causes et sur les limites des variations de structure des veg^taux. M. J. Vesque, in 
Ann. agronomique, vol. ix, pp. 495-6. 
