852 Yapp . — Spiraea Ulmaria , Z., and its Bearing on the 
afforded by the intermediate, partially hairy leaves of Spiraea. In these 
the leaflets, and even the portions of leaflets, which are most exposed to the 
danger of excessive loss of water, are provided with a dense covering of 
hairs (see PL LXXXII and LXXXIII). 
Bearing in mind all the facts adduced in this paper, it is difficult to 
avoid the conclusion that the production of hairs in Spiraea and certain 
other plants, under the stimulus of drought, is of the nature of an advantageous 
adaptive response. The most important physiological effect, then, of this 
response would seem to be to prevent undue acceleration of transpiration 
by air movements and, though to a less extent, insolation. 
Turning once again to the question of root-hairs and palisade cells, the 
chief role of the former (i. e. to increase the absorbing surface of the root) 
seems clear. The functions of palisade cells, on the other hand, have 
been much disputed. Probably they perform several important physio- 
logical functions. The only view which need be mentioned here is that of 
Areschoug, 1 who regards a well-developed palisade tissue as of importance 
in reducing transpiration. Areschoug’s view has been accepted by many 
authors, though so far direct experimental evidence is lacking. 2 
Granted the foregoing, it appears that the stimulus of a decreased 
supply of water promotes the formation of three different kinds of elongated 
cells, each of which is physiologically important (at least in certain cases) 
in connexion with the plant’s water supply. At one end of the plant the 
root-hairs increase the intake of water : at the other the hairs and palisade 
cells assist in reducing the output of water from the leaves. There would 
thus seem to be in these cases, as in many others, a more or less definite 
connexion between the nature of the stimulus and the physiological 
advantage of the response. 
§ 13. The Xeromorphy of Spiraea Ulmaria . 
Spiraea Ulmaria is far from being a pronounced xerophyte. Its leaves 
are thin and frequently of considerable size. They have a fairly thin cuticle, 
and generally wilt readily when gathered. Nevertheless the plant exhibits 
certain xerophytic characters which are practically confined to the upper, 
more exposed parts of the shoots. These characters are as follows : 
1. The lower stomatal surface is covered by a dense pubescence. 
2. The upper epidermal cells have a better developed cuticle than 
those of the lower leaves. 
3. The mesophyll is much more compact ; the internal transpiring 
surface being thereby considerably lessened. 
1 Areschoug (’ 82 ), p. 520, and (’ 06 ), p. 329 et seq. 
2 Hesselman (’ 04 ), p. 428, however, as a result of certain field experiments, is of opinion that 
palisade tissue does not markedly depress transpiration. Some of his experiments were actually 
carried out on Spiraea Ulmaria. Areschoug has discussed Hesselman’s objections in his 1906 paper, 
