THE SHAPES OF LEAVES. 



157 



where in the Victoria regia there is merely a seam. Among 

 land-plants similar forms are found under analogous condi 

 tions. The common Hydrocotyle, Fig. 219, which sends 



up direct from its roots a few almost upright leaf-stalks, has 

 these surmounted by peltate leaves; which leaves, however, 

 diverge slightly from radial symmetry in correspondence with 

 the slight contrast of circumstances which their grouping in 

 volves. Another case is supplied by the Nasturtium, Fig. 

 220, which combines the characters a creeping stem, long 

 leaf-stalks growing up at right angles to it, and unsymme- 

 trically peltate leaves, of which the least dimension is, on 

 the average, towards the stem. But perhaps the most 

 striking illustration is that furnished by the Cotyledon umbi 

 licus. Fig. 221, in which different kinds of symmetry occur 

 in the leaves of the same plant, along with differences in their 

 relations to conditions. The root-leaves, a, growing up on 

 vertical petioles before the flower-stalk makes its appearance, 

 are symmetrically peltate; while the leaves which subse 

 quently grow out of the flower-stalk, 6, are at the bottom 

 transitionally bilateral, and higher up completely bilateral. 



That the bilateral form of leaf is the ordinary form, 

 corresponds with the fact that, ordinarily, the circum 

 stances of the leaf are different in the direction of the plant s 

 axis from what they are in the opposite direction, while 



