180 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



taken into account, the substance of the leaf is seen 

 to be disposed with a view to promote evaporation. 

 One applies the same principle when his coat has 

 got wet. It would take a long time to dry if it 

 were tightly rolled up into a bundle, compared with 

 the time necessary if the coat be spread out and 

 hung up. When a plant grows partly in water and 

 partly in air there is a remarkable difference 

 between the submerged leaves and those produced 

 on the aerial part of the stem, the former being 

 divided into linear thread-like segments. The aerial 

 leaves of Ranunculus heterophyllus bear the same 

 relation to its submerged dissected leaves that the 

 web-foot of a duck does to the toes of a hen. The 

 dissected leaf has the advantage of offering less 

 resistance to water currents. 



2. The Discoid Form. — An organism originally 

 spherical may depart from that shape in various 

 directions. It may pass through the ellipsoid or 

 ovate form to the conical and cylindrical. As 

 instances of the cylindrical, the rod-like Bacteria 

 and the filamentous Confervce may be mentioned. 

 To this type we may also refer Chara, Equisetiun, 

 Juncus, and the trunks of most trees ; but it seems 

 probable that in the discoid forms of many Lichens 

 and AUjce we have a more primitive type. If, in the 

 developing oospore, we suppose that growth, instead 

 of taking place equally in every direction, is restricted 

 to one plane, and, instead of being dominated by a 

 single axis, occurs in the radii of a circle, then there 

 is produced, from the original sphere, a flat circular 

 or plate-like form. This disc shape is unobjection- 

 able so long as the jDlant can grow appressed to 

 soil or rocks. Where, however, the plant cannot 

 command a large enough free space, it will be 

 forced to assume a vertical position. In such cir- 

 cumstances it is impossible for the plant to attain any 

 very considerable dimensions while continuing to re- 

 tain its circular form. For one thing, the resistance of 

 the myi\ of sea-bed will prevent the development of 



