THE ORCHID REVIEW. 107 
I had frequently examined these vertical roots at Kew, but was unable to 
account for the peculiar arrangement. The case of mangrove, however, 
where the main roots are in the water or mud, suggests the probability that 
they are developed in order that the plants may feed on the gases in the air, 
and Orchids might find a similar benefit in getting away from the accumula- 
tions of vegetable matter in which they grow into the free air. The vertical 
arrangement may be partly due to the roots acting as condensers of the 
water in the air, and thus collectors of the gases present, and as the young 
absorbent point is at the apex it is in a position to benefit immediately. 
This is suggested as a possible explanation, but in any case, it is evident 
that these vertical roots have the same function as those much-branched 
aerial roots of Rodriguezia, Ionopsis, and other Orchids, which obtain their 
food entirely from the air, not being in contact with any other substance 
whatever. Here again we see the importance of imitating natural conditions, 
for such roots will not remain in the compost, but will grow out again as 
often as placed there. 
Certain aerial roots contain chlorophyll, or the green colouring matter of 
leaves, without which nutrition of the plant cannot be carried on, and the 
roots of those curious leafless Orchids, previously mentioned, show a 
remarkable change of function in this respect. First comes the change 
from an ordinary root to the modified aerial condition, and then the develop- 
ment of chlorophyll, by which the plant is able to assimilate its food and 
thus dispense with ordinary leaves altogether. In these plants aerial roots 
have not only to collect the food of the plant, but to assimilate it too, and 
in this they succeed very well, for examples are found in each great tropical 
area—certain species of Angraecum in Africa, Dendrophylax and Campylo- 
centrum in America, Tzniophyllum and Sarcochilus in India and Malaya. 
A species of the latter genus, S. luniferus, succeeds very well at Kew, and 
flowers every year. 
So much for the question of food supply, which is certainly a very impor- 
tant one. I believe that some Orchids which under cultivation gradually 
dwindle away are literally starved to death. The necessary amount of food 
is either absent or is supplied in a form which they cannot assimilate, either 
of which must be fatal in the end. The inflorescence of certain Orchids, as 
well as the bulk of the plant, seems altogether disproportionate to the 
amount of compost used, and this of itself suggests the probability that much 
of the food is absorbed from the air or from the water supplied. Rain water is 
invariably recommended for Orchids rather than spring or well water, and 
although the presence of lime in the latter is often assigned as a reason, it 
may be partly due to the presence of ammonia in the former. Layers of 
oak and other leaves are put under the stages by some growers, -_ 
sprinkled with water occasionally, and it is easy to see how the gases given 
