204 Gibbs . — Bio-histological notes on some new 
also within the whole alliance. In the Acanthaceae tuberous development 
is very rare, and Lindau 1 only refers to Ruellia tuberosa as an example, 
in which case it takes the form of local swelling of the roots. The peculiar 
development of specialized leaves for nutritive purposes finds no parallel 
in this order, though rooting at the nodes is a general feature. The 
production of resting buds is well known in certain families, notably 
species of Epilobinm , i. e. E. parvifolinm Shreb., E. montanum Linn., and 
E. lanceolatum Sebast. et Maur, where the autumnal stolons produce 
fleshy white rosulate leaves, very similar in appearance to those occurring 
in the present case. Goebel 2 quotes Androsace sarmentosa Wall, as forming 
resting leaf rosettes consisting of leaves very different from the foliage- 
leaves. In U tricularia and Myriophyllum winter buds or turions are formed, 
but these examples serve also for vegetative reproduction, as they may be 
detached from the parent plant, which is not the case with sessile leaf 
rosettes, which are organs modified as reserved storage-tissue for the purpose 
of accelerating next year’s growth. 
In conclusion we may point out that in two plants of most widely differ- 
ing organization and systematic position, exposed to the same physiological 
but rather divergent edaphic conditions, the direct response is on the same 
lines in both cases, and in the same direction, viz. to the general or physio- 
logical stimulus rather than to individual requirements. The important 
organogenic modifications involved are attained in both cases by the speciali- 
zation of some organ or organs as a reserve food store, thus providing not 
for present requirement, but for future need. To ensure next season’s rapid 
growth on the most economical lines, present opportunities are utilized to 
the utmost, but the necessity of elaborating contrivances to meet potential 
adverse circumstances represented by a prolonged physiological drought 
is discounted, and the plant secures itself against the periodic recurrence 
of dry seasons by the cessation of all vegetative activity in the annual dying 
down of its aerial shoots. It is the mean of physiological environment 
and not the extreme which here determines biological modification. In 
the case of Fuirena Oedipns y which was in flower in September, the 
favourable growing period would probably cease in January, when the 
summer rains begin to affect the volume of water in the river (p. 291). 
From then till June it would be exposed to a continual downpour, with 
its rhizome and roots in standing water. For Justicia elegantida the 
conditions are apparently different, though physiologically the same, as it 
is one of the few surface-rooting veld plants. Amongst these latter the 
absence of stolons or surface-rooting runners was very conspicuous, though 
a widely spreading system of dorsiventral shoots was such a very general 
adaptation. Listia heterophylla E. Meyer, on moist sand-banks near streams, 
roots very freely at the nodes, but the same plant on the veld shows a very 
1 Lindau, Nat. Pflanzenfamilien, iv. 3 b. 2 Goebel, Organography of Plants, p. 398. 
