846 Yapp.— Spiraea Ulmaria , Z., and its Bearing on the 
probable that the upper part of a normal Spiraea shoot has, during develop- 
ment, a less constantly maintained turgor than the lower , 1 and that the 
water supply is the limiting factor in the growth in size of the cells. 
But now arises a difficulty. Although turgor is no longer regarded 
as the direct cause of the stretching growth of cell-walls , 2 it is none the 
less a condition of such growth . 3 It is difficult to conceive how either 
hairs or palisade cells can elongate unless they are turgid. And yet we 
have seen that they are most strikingly developed under conditions when 
turgor may be expected to be at a minimum. The difficulty, however, is 
more apparent than real. In nature the external conditions are never 
constant. No matter how active transpiration may be at certain times, 
periods frequently recur (e. g. at night-time or on still, humid days) when 
turgor is fully restored. 
The course of events then is probably somewhat as follows : transpira- 
tion during the day-time, increasing with the number of leaves unfolded, and 
(at least until midsummer) with the march of the seasons, gives rise to 
a diminished turgor in the cells of the developing leaves. This disturbance 
of the water supply to the yet plastic cells acts to certain of them as 
a stimulus (merely, of course, an initial or proximate one), in response to which 
the cells assume an elongated form . 4 But the actual stretching growth of 
the cells can only take place when they once more become turgid. This 
occurs when transpiration falls at night-time, &c . 5 
Boodle , 6 in his suggestive paper on the Bracken Fern, has made 
a similar suggestion with regard to the factors determining leaf structure. 
He says ‘ it may possibly be some such factor as strong periodic fluctuations 
in the turgescence of the cells of the leaf (due to scarcity of water when 
transpiration is most active) which determines in the immature leaf of the 
Bracken whether the leaf shall be xerophytic or not ; and the same may 
apply to other plastic species \ 
The case seems clearly one of stimulus and response, and the some- 
what mechanical theories referred to above appear to be inadequate. 
1 Cf. Pfeffer (’03), p. 120. Miss Delf (’ll), p. 501, states that the upper internodes of Salicornia 
had not only more numerous stomata than the lower, but also much smaller epidermal cells. This 
she regards as indicating that neither they nor the guard cells had attained their full size; and this 
notwithstanding the fact that the stomata were fully formed and open. Whatever the case may have 
been in Salicornia , the upper leaves described for Spiraea were certainly mature, in spite of the 
small size of their cells. 
2 As Wortmann (’89), p. 293, &c., and others supposed it to be. Cf. the discussion of this 
question in Sokolowa (’98), p. 179 et seq. 
8 Cf. Pfeffer (’03), p. 118 et seq. ; also Sachs (’87), p. 565. 
4 In this case the stimulus appears to induce the cells to assume a special form . In many of the 
cells of these upper leaves it is chiefly the size of the cell which is affected, owing to the stretching 
growth of the cell-walls being less. 
5 It seems certain that such fluctuations in the moisture content of leaves do actually occur, at 
least in mature leaves. Cf. Thoday (’09), p. 19 et seq., also Livingston and Brown (T2). 
6 Boodle (’03), pp. 665-7. 
