Problem of Xeromorphy in Marsh Plants. 843 
also may to some extent be included in the parallel. Probably all three 
elements play an important part in relation to the water supply of the plant 
(cf. § 12). They may therefore be considered together, though the main 
concern of this paper is with the leaf-hairs of Spiraea. 
In the first place, in so far as the plant is plastic, is it possible to deter- 
mine the nature of the stimulus to which the plant responds by the develop- 
ment of these special elongated cells ? 
Kraus 1 explained the formation of hairs on potato stems in dry air by 
supposing that decreasing humidity hindered growth in length. In con- 
sequence, said he, turgor increases in the transverse direction, and the 
epidermal cells grow out in the form of hairs. 
Mer 2 supposed that the appearance of both stem- and root-hairs is due 
to the accumulation of nutritive substances caused by a slowing down of 
growth. Vesque 3 adopted Mer’s view, though he thought it complicated 
matters to include root-hairs in the hypothesis. 
Jungner 4 suggested a theory respecting the formation of hairs on 
certain xerophytes of alpine regions, extending it to include those of deserts 
and salt steppes. Although acknowledging that the plants are exposed to 
strong evaporation, he assumed that transpiration is depressed during the 
early stages of leaf development. Consequently the pressure of turgor in 
the cells is high, and, the air outside being rarified, the turgid cells grow 
out in the direction of least resistance. 
Turning now to theories of palisade formation, Stahl 5 supposed that 
intense light retarded the growth in length of the veins of developing 
leaves. The young assimilating cells then expand in the only direction 
possible (i.e. that perpendicular to the leaf surface), and in consequence 
assume the form of palisade cells. 
Mer G attributed a marked development of palisade cells to active 
nutrition resulting from the action of light in dry air. 
Vesque 7 regarded transpiration as the chief factor inducing the forma- 
tion of palisade tissue. Kohl, 8 who held the same view (and applied it also 
to hair formation), further pointed out that transpiration affects turgescence, 
and turgescence regulates the growth of cell membranes. 
Of these theories, that of Kohl (and of Vesque so far as palisade cells 
are concerned) agrees best with the known facts (cf. § 9). All the evidence 
available suggests that any marked development of these peculiar elongated 
elements is in some way associated with decreased turgor rather than the 
increased turgor postulated by Kraus and Jungner. This evidence is as 
follows : 
1. The experiments of many observers , which show that in general these 
1 Kraus (76), p. 153. 2 Mer (’83), p. 115. 3 Vesque (’83), p. 495. 
4 Jungner (’94), pp. 233-4. 5 Stahl (’8 3), p. 34. 6 Mer, 1. c., p. 112. 
7 Vesque, 1. c., p. 489. 8 Kohl (’86), pp. 95 and 103. 
