142 



MORPHOLOGICAL DEVELOPMENT.. 



where in tlie Victoria regia tkere is merely a seam. Among 

 land-plants similar forms are found under analogous condi- 

 tions. Tlie common Hydrocotyle^ Fig. 219, wliicli 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 circimistances 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 unsjTnme- 

 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 unihi- 

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

 in the leaves of the same plant, along '^dth differences in their 

 relations to conditions. The root-leaves, a, that grow 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, b, 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 



