PHYSIOLOGY. 115 



diameter covered with the foliage from a single well-grown specimen. If 

 the gardeners have not been too careful we see that the outer border of 

 this ring is occupied by yellow and decaying leaves on very long petioles. 

 Within these comes a zone of large green and shining healthy leaves, then 

 smaller tender ones, and, at the center, one or two are only partly unrolled 

 from the bud. The rate of elongation of a petiole of N. rubra detached 

 from the parent plant was 2.5 cm. per day. This, with perhaps a certain 

 amount of stretching of the collenchyma and softer tissues, may account 

 for the rise of leaves on plants moved from shallow to deeper water, but 

 it does not reach the case of adjustment from deep to shallow water. 

 Frank (1872) investigated the causes of this phenomenon, giving especial 

 attention to Hydrocharis morsus-ranae, and concluded that gravitation 

 (as expressed by water pressure) and light, though not the causes, are 

 used by the plant as signs by which to measure how much it has yet to 

 accomplish before a growing organ has reached its most advantageous 

 position (" Gravitation und Licht sind nicht die Erreger jener Wachsthums- 

 formen, sondern die Pflanze bedient sich ihrer nur als Merkmale, an denen 

 sie abmisst, wieviel sie noch zu leisten hat, bis das durch Wachsthum zu 

 richtende Glied seine vortheilhafteste Lage erreicht hat." p. 85). The 

 expression is an excellent one, even if the " signs " are insufficient. For 

 is it not so with all stimuli and all organisms ? No external stimulus is in 

 itself a final cause of any physiological action ; that is, the irritability of 

 protoplasm is the indispensable condition of all vital functions. Karsten 

 (1888), experimenting on Hydrocharis and Ranunculus scelcr'atns, showed 

 the insufficiency of Frank's explanation, and proved that it is the oxygen of 

 the air which inhibits the growth of the petioles as soon as the leaf-blade 

 reaches the surface of the water ( " dass es der Sauerstoff der Atmos- 

 phare ist, welche bei den Schwimmblattern jene constatirte Hemmung im 

 Wachsthum ihrer Stiele bewirkt, sobald sie die Wasseroberflache errei- 

 chen." p. 577). In Nymphaea, however, the inhibiting influence of the 

 air still permits the slow elongation of the petioles by which the older 

 leaves are moved farther and farther away from the center of growth. 



Once spread out upon the water surface, the leaves are subjected to 

 the stress of currents in air and water. Jahn (1896) pointed out four 

 requirements for such foliage: (i) lightness and firmness of lamina; (2) 

 the greatest possible amount of surface ; (3) insertion of petiole at center 

 of leaf; (4) the petiole must make a large angle with the lamina. The 

 first is attained by the great amount of air space, and in Lotos and Apo- 

 carpiae by the girder-like veins. But in strong winds the leaves are often 



