864 Yapp . — Spiraea Ulmaria , Z,., #7^ zZs* Bearing on the 
and (c) external conditions. It is shown that evidence of many kinds 
points to hairiness being associated with a diminished, and glabrousness 
with an abundant supply of water (see 6 and 7 above). In so far then 
as Spiraea is plastic in respect to hairiness, the course of events would 
seem to be somewhat as follows. As transpiration increases with the 
unfolding of the leaves in spring-time, the amount of water available for 
the apical bud becomes diminished. Certain cells of the developing leaves 
are thereupon stimulated to form hairs. This is a case of what Wiesner 
has called ‘ correlative transpiration ’. When the partially developed leaf 
emerges from the bud, its supply of water is still further lessened by 
cuticular transpiration from its surface (cf. 8 above). The initial stimulus 
which leads to hair formation is thus afforded by diminished turgor ; 
the actual growth in length of the hair cells, however, cannot take place 
till turgor is once more restored. This takes place at more or less regu- 
larly recurring intervals, especially at night-time. Thus hair production 
is promoted by marked periodic fluctuations in the turgor of the hair- 
producing cells. Spiraea Ulmaria possesses only a limited degree of 
plasticity in regard to hairiness. It is suggested that the species is tend- 
ing to become fixed, in the direction of response to the different conditions 
to which the upper and lower leaves respectively have been exposed in 
nature for many generations. 
10. With regard to the physiological effect of the hairs, &c., it is 
concluded that the pubescence prevents undue acceleration of transpira- 
tion, and especially that due to air movements and, to a less extent, insola- 
tion. It is probable that the stimulus of a decreased supply of water during 
development promotes the formation of three special kinds of elongated cells, 
each of which is physiologically important in connexion with the problems 
of water supply. At one end of the plant the root-hairs increase the 
intake of water, and at the other the hairs and (probably) the palisade 
cells assist in reducing the output of water from the leaves. 
But the experimental treatment of the subject of the effect of leaf 
structure on transpiration is reserved for a future paper. 
11. Structurally, the leaves of Spiraea Ulmaria show considerable 
differences according to position and time of development. The lower 
sheltered leaves are hygrophytic, while those which are exposed to wind, 
strong insolation, &c., are distinctly more xerophytic in character. It is 
concluded that this degree of xeromorphy of the upper leaves (hairiness, 
&c.) is a positive advantage to the plant, and probably actually necessary, 
at least under unusually severe transpiration conditions. 
12. The whole position of our knowledge of ‘swamp xerophytes 5 
is reviewed, and the conclusion arrived at that all the evidence points 
to the various bog and marsh associations being as much in harmony with 
their environment as are those (e. g. woodland, desert associations, &c.) 
