TRANSACTIONS OF SECTION K. 801 
In conclusion, I should like to add to the above-mentioned cases others of 
disjointed distribution which seem to be the result of other evolutions than those 
of the species considered so far: Canarina abyssinica, Engl., from Gallaland ; 
C. Emini Aschers, from Ruwenzori mountain; C. campanula, Lam., of the 
Canary Islands; seem to indicate that Canarina is an older genus, whose species, 
having travelled in more remote periods, may have formerly had wider areas; 
perhaps it was indigenous even to the Mediterranean region, like the genus 
Sempervivum, sect. Aconium. 
With regard to the species or varieties I have enumerated, I may be allowed 
to make a generalremark. One can call such modifications adaptations, but only in 
this sense, that the adaptation is a passive one, caused by the physical conditions of 
the climate; not an active one, corresponding to Lamarckian views. 
6. Mechanical Advantage among Plant Organs. 
By J. CuarK, Ph.D., BSc. 
Experiments with the asymmetric leaves of certain Commelinacex plants 
confirmed the view (first published by Spencer and endorsed by Goebel) that light 
is a factor in bringing about their asymmetric form. Closer examination, however, 
showed me that difference in light intensity is a very improbable explanation of 
this asymmetry, especially as by careful measurement another explanation pre- 
sented itself, in which light is an indirect factor. 
It became evident that the asymmetric leaves (which only occur on dorsiventral 
shoots) are differently placed with regard to the stem which bears them, from 
those on radial shoots whose leaves are quite symmetric. The former, in fact, 
undergo a twist, whereby one side of the leaf is drawn towards the stem, thus 
bringing about a ‘hemmage’ of the sap current to that side of the leaf. By 
exciuding the light factor, it is possible to obtain symmetric leaves on dorsiventral 
shoots, for in this case no leaf-twist takes place. 
Wider Application of the Mechanical Advantage Hypothesis. 
1. Other cases of Asymmetry (Begonia leaf for example) are clearly explainable 
by the above view, for the larger side of leaf is always most directly continuous 
with the leaf-stalk. 
2. Anisophylly.—The upper leaves on many plagiotropous branches are smaller 
than the lower leaves, because, while obtaining a favourable light position, the 
former must twist into an angle of disadvantage with regard to the sap current. 
8. Abnormal Palmate Leaves.—The leaflet in most direct communication with 
the petiole is in the most favourable position for growth. 
4, Mr. Zeleny’s experiment with palmate leaves. 
5. Illustration showing the probable origin of peltate leaves through rotation 
of leaf lamina into a position at right angles to its petiole. 
6. Substitution of a main by a side shoot. 
7. The double-U form obtained by gardeners. 
8. Preference of side roots for convexities. 
Besides offering a very probable explanation of many interesting phenomena in 
lant life, weighty evidence is brought forward against that view of plant growth and 
its relation to plastic substances which recognises a mere ‘ wandering of plastic sub- 
stances,’ and endorses the view of Sachs when he says: ‘Die Anregung zu den 
Stoffbewegungen aber wird immer durch das Wachstum der jungen Organe 
gegeben ; die Knospen eines Baumes treiben im Friihjahr nicht etwa deshalb aus, 
weil, wie die Leute sagen, der Nahrungssaft in sie eindringt, sondern gerade 
umgekehrt: die Nahrungsstoffe werden in Bewegung gesetzt, weil die Knospen zu 
wachsen anfangen.’ 
7. Exhibition of Pure Cultures of Alge. By Professor R. Cuopat. 
1904, 3F 
