DEVELOPMENT OF SIUM CICUTAEFOLIUM. 13 



Besides these differences in form between the senescent and the juve- 

 nile leaves, there is quite as marked difference in the texture, venation, 

 etc. The dissected leaf has already been spoken of as having a form 

 not rarely associated with aquatic habitats. Similarly the juvenile and 

 senescent leaves have forms and textures usually associated with meso- 

 phytic and xerophytic habitats, respectively, the reduction of the blade, 

 firm texture, and strong cuticularization of the stem-leaves being in 

 marked contrast with the broad blades and delicate texture of the juve- 

 nile leaves. It will not be surprising to find that each of these three 



FIG. 3. The senescent series of leaf-forms in Sium cicutae folium. 

 Numbered downward from n, the most reduced leaf found in 

 the inflorescence; y a terminal leaflet showing incomplete 

 differentiation of a pair of lateral leaflets. Two-thirds natural 

 size. 



types of leaves is associated with just the conditions of environment to 

 which it would appear to be adapted. The seedling develops in the 

 shade of other plants during the summer, when the substratum is less 

 wet ; the dissected leaves occur in autumn and spring, when the favor- 

 ite habitats of the plant are apt to be supersaturated or flooded with 

 water; and the stem-leaves are raised above the substratum, where 

 they are more exposed to the drying influences of wind, light, etc., and 

 at the same time farther from their water-supply. 



But too much stress can easily be placed upon external appearance 

 as evidence of adaptation, and the dissected leaves, which suggest by 

 their form an adaptation to hydrophytic conditions, do not bear out the 

 suggestion when put to the test. They are unable to do sufficient pho- 



