4 
Proceedings of the Koyal Society of Edinburgh. [Sess. 
successive transverse zone from the juvenile to the adult region. It may 
be anticipated that at some point of size a critical proportion of surface 
to bulk will be reached, where the interchanges between stele and cortex 
will demand some alteration of structure if they are to be satisfactorily 
carried out. 
Conversely, however, an axis or root may diminish progressively in 
bulk from the base upwards. In a fern that has been starved by 
unfavourable culture the size of its stem is less than in the region 
developed under normal conditions, and the internal tissues follow suit, 
with simplification of structure. Certain strut roots of palms develop 
lr 
1 , 
thinner after entering the ground than 
above it. These should then show pro- 
gressively converse structural changes, if 
the reasoning be correct. For them the 
problem, so far as it depends upon size, 
would be progressively simplified, and the 
evidence of this might be expected to 
appear in their structure. It will be 
shown later that this expectation is justi- 
An illustration already familiar to 
Fig. 2. — Median longitudinal section fied by the facts, 
through the prothallus and embryo 
of Polypodium vulgare. x 6. Leaves 
A h> et( L roots, op = apex of botanists of the way in which form may 
stem. the drawing shows the J J 
widely expanding conical stem— be modified so as to secure an increase of 
small at the base, where it is pro- 
tostelic; larger above, where it is surtace-area, and so to facilitate transit 
dictyostelic. through that surface, is seen in the case 
of submerged leaves. In Hottonia, Potamogeton pectinatus, Ranunculus 
aquatilis , and Gabomba, etc., the submerged leaves are cut into narrow 
segments, differing thus in marked degree from the undivided blades of 
the aerial leaves of allied plants. In the cases of Gabomba and R. 
aquatilis {heterophyllus) the difference appears even in the leaves of the 
same plant. On the other hand, in Ouvirandra the submerged leaves 
are perforated by many oblong holes. The biological reason for these 
peculiarities is to be sought in the fact that, by being thus subdivided 
or perforated, they expose a relatively large absorbent surface to the 
water, out of which they abstract the materials for their food. In par- 
ticular, oxygen and carbon-dioxide are exchanged with the water through 
the epidermis, which has here no stomata, so that their external surface 
is the only available surface for the purpose. In this respect their struc- 
tural difference from the entire and slightly lobed aerial leaves which bear 
stomata makes an increase of absorptive surface all the more necessary. 
