500 Uphof. — Contributions towards a Knowledge of 
that of the incident light ; when the darkest part was on the left of the 
stem, the aerial roots turned themselves in this direction ; on the other hand, 
the true shoots always tend to grow positively heliotropic. Of old or partly 
full-grown rhizophores, the growing-point always develops towards the 
darkest side again after having been moved. 
Diversified directions of various aerial roots can be readily observed 
on strongly branching Selaginellas, which grow intermingled with other 
densely growing plants and thus do not get an equal distribution of light. 
On such plants one finds these roots growing in all directions from vertical 
to horizontal, according to the effectiveness of the entering light. 
Plants which were grown in hanging baskets in a light part of a green- 
house show in their rhizophores various degrees of sensitiveness to light. 
Rhizophores or aerial roots which are hanging from the stem between the 
glass of the greenhouse and the basket in which the plant grows are strongly 
curved towards the basket ; on the other side, where the light is less intense 
on account of the presence of other plants, the curvature of these aerial 
roots is less marked ; thus the degree of the sensitiveness is not so pro- 
nounced, and consequently the rhizophores are less curved in the direction 
opposite to the light. The above facts were observed by the author with 
5. uncinata , vS. Lobbi , 5. W atsoniana y and .S'. Martensi. In a practically 
similar way the negative heliotropism was demonstrated by putting parts of 
stems with young rhizophores of .S', grandis , 5. serpens , 5. haematodes , and 
5. Bakeriana in watch-glasses which were covered with a glass plate with 
black paper above it except for a narrow margin towards the window side 
of the laboratory in which they were placed. The top of the rhizophore or 
aerial root was placed in such a way that it was touched by the light. After 
eight days the apex of these rhizophores had all partly developed in the 
opposite direction to that of the incident light, whereas the young shoots 
were growing towards the light. 
When aerial roots enter the ground, their one-sided growth is not as 
strongly marked as if they were growing in the air, as the opposition of 
the light is overcome as soon as they enter the soil. Their growth is 
more or less spreading in all directions, depending only upon the moisture 
and the presence of plant food in the substrata. In the soil they branch 
monopodially in the same way as they would do in the air. 
Aerial roots which are exposed to the light contain in their cells a con- 
siderable amount of chloroplasts, which are absent in the roots growing in 
the soil ; such organs of »S. Wildenowi y S. rubella , and 5. Lobbi y which were 
afterwards exposed to the light,* became greenish and developed chloro- 
plasts, especially in the cortex. 
Some anatomical difference exists between the aerial root and the 
terrestrial root ; the former, on account of its environment, is surrounded by 
a thick-walled epidermis and hypodermis, and sometimes one or two layers of 
