IN THK M.>1 OR OBGANS DP LEAVES. 



6:> 



their opponents, but are also as a rule specially richer in ehloro] >h v 11 , and therefore 

 penally subject to the influence of solar simulation as a source of increased 



^ .nco 



dependent on the manufacture of a-imilatory products. This in many rases, as in tli 



of the pinna! pulvini of Gtmm data, and in the second:; y :md tc iiuy pulvini of Pith 

 colobtum saman, is evident to the most cursory inspection owing to superficial differences 

 in colour, and in all it can ho readily ascertained by means of micms ( - pic senior 

 especially if th. <e have been momentarily immei ed in a 2 per (cut. solution of comic 

 acid so as to fix the colouring matter in the chromutophores. (Mate I. Figaro 19; 

 Plate II, Figs. 3, 5—8: Plato IV, Figs. 3. 5 



The diurnally dominant tissues in the pulvini of nyctitropic leaves are then 

 distinguished from the noetnrnally dominant ones by precisely the sam, feature.-* 



whi.h distinguish the guard-cells of tho stomatic orifices from the common <-pid nnal 



elements. 







They are distinguished by their youth, i.r by the large proportion () f 



on or re 



young elements which they contain, by their relative structural weakness, and bv their 

 relative functional strength, especially in relation to solar tiinulation; and tl 



ie move- 





ments which they give rise to are of essentially similar origin to those caused by 

 the guard-cells. Just as the guard-cells are able to eff( - 1 a displacement of the 

 surrounding tissue when exposed to sunlight because ..f their temporary excess in 

 turgescence, so are the masses of tissue in tho pulvini which make for the diurnal 



position able to effect displacement of their opponents under similar eircumstaii- «. 

 The condition of the stomatic orifices is regulated not merely by tl actuations in tie 

 degree of solar stimulation to which the tissues are exposed, but also by the coincident 

 conditions of root-supply and transpiiatory loss of water; and the same holds good b 



regard to the position of nyctitropic leave.. Where tra n s j >iratory loss is in x< is, 



and yet falls short of the degree noce-ary to cause general wilting of the tissues, 



we find both stomatic orifices and ny c ti t ro pic leaves assuming the nocturnal position ; 

 and in both cases a resumption of the diurnal one follows a restoration of the 

 normal relation between general loss and supply of fluid, whether this be attained 

 ' means of diminished transpiration or increased root-sunply. The excess of 

 functional activity in the guard-cells, and the masses of pulvinar tissue making for the 

 diurnal portion, provides that they shall undergo a greater increase in turgescence 

 under solar stimulation than their opponenti do, anl this implies a temporary ino ase 

 in the resistance which they present to them; but at the same time the excess of 

 structural strength in the common epiderm il e!em nts and the pulvinar tissues making 

 for the noctural position gives them a relative protection from the action of conditions 



implying an excess of transpiiatory loss over root-supply of water. The result of 



this naturally is that, where this excess rises high enough, a resumption of the 

 nocturnal position occurs in spite of the continued stimulation of the more active tissues, 

 because these are no longer able fully to satisfy their osmotic capacities. Where 

 conditions of general supply and loss of fluid hold a normal relation to one another the 

 diurnal position is secured by the greater osmotic Capacity of the more active tissues* 

 but as these are also structurally weaker ones, they are tho first to suffer from loss 

 of turgescence connected with insufficient supply. 



Stomatic movements are specially dis?ingui>hed by their persistence and constancy 

 of character. Once established, they continue to occur with constancy and regularity 



