IXKU'KXrl''. OK LICHT. 505 



six young plants, passed the winter under a thick coat of snow in the open. 

 At the beginning of December the six plants in the hot-house sent up from 

 the centre of their small leaf-rosettes slender stalks 10 cm. high, whose upper 

 internodes were 22 mm. long and 1 mm. thick. The stem-leaves were yellowish, 

 entire, elongated, 6-7 mm. long and 2 mm. broad; calyx-tube 4 mm. long, 13 mm. 

 broad; calyx-teeth 2 mm. long, 15 mm. broad; petals 35 mm. long, 2 mm. 

 broad; stamens 1 mm. long. It was noted that lateral axes only developed in 

 the axils of the upper stem-leaves, and that the buds of the lateral shoots in the 

 lower leaf-axils atrophied. In the following May strong stems G cm. high were 

 sent up from the leaf-rosettes of the plants which had wintered under the deep 

 snow in the open; their upper internodes were 12 mm. long and 2 mm. thick. 

 The stem-leaves were somewhat broadened in front with dentate margin, red in 

 colour, 5 mm. long and 3 mm. broad. The measurements of the parts of the 

 (lowers were: — Calyx-tube, 2 mm. long, and 2 mm. broad; calyx-teeth, 1*5 mm. 

 long, 1 mm. broad; petals, 2'3 mm. long, and 2 mm. broad; stamens, 1 mm. long. 

 From the axils of the stem-leaves flower-bearing shoots developed, which, like the 

 parts of the main stem exposed to the sun, were coloured red. Here then the 

 alterations which certainly are due to the various light influences consist not only 

 in the lengthening and shortening of the stem- and foliage-leaves, but the flowers 

 are correspondingly changed. The petals of the flowers which opened at the New 

 Year when the days were shortest were not only relatively but actually narrower 

 than those which belonged to flowers which opened in the early summer when the 

 days were longest. 



It has already been stated that the elongation of the leaves and the division of 

 the leaf-lamina into long narrow segments in submerged leaves is associated with 

 the diminution undergone by the light in passing through the water (see vol. i. 

 p. 665). The elongation of submerged leaves is very well seen in the water Star- 

 wort (Callitriche) and Mare's-tail (Hippuris). In the latter the linear submerged 

 leaves are thirty times as long as they are broad, while the length of the aerial 

 leaves is only 7-9 times their width. In Roripa amphibia the leaves which 

 develop under water are deeply cleft compared with those produced in the air. 

 The aerial leaves of this Crucifer are linear-lanceolate, about ten times as long as 

 broad, with finely toothed margin. Under water the leaves have an elliptical 

 shape, are 2-3 times as long as broad, and the lamina is cleft almost down to the 

 midrib in narrow segments 2-3 cm. long, like a comb or feather. The aerial 

 leaves of the whorled Waterwort (Elatine Alsinastrum) are grouped in whorls of 

 three. They have an ovate shape, and their margins are finely notched. Each is 

 traversed by 3-5 veins. The leaves developed under water are divided almost 

 their whole length into 3-4 narrow linear segments, and each wdiorl looks as if it 

 were composed of twelve leaves. Each segment is smooth round the edge, and 

 traversed only by one central vein. The difference between the aerial and sub- 

 merged leaves of the white-flowered Crowfoots (belonging to the Batrachium 

 section of the genus Ranunculus) is even more surprising. Plants of these Crow- 



