EXTERNAL CONFIGURATION OF PLANTS. 181 



the lower semicircle. The plant will thus be reduced 

 to a semicircular or fan-like form, examples of 

 which are to be found plentifully among marine 

 Alqce. Even with this fan shape, however, no great 

 individual development is possible, because it would 

 oifer too great mechanical resistance to water 

 currents if the plant were aquatic, and to the Avind 

 if it were terrestrial. To obviate this defect we have 

 a further adaptation. 



3. The Digitate Form.— If growth do not take place 

 along every radius of the fan, but be restricted to a 

 few radii, then the digitate or hand-shaped frond 

 results, which has the advantage of greatly reducing 

 the resistance to wind and waves. This is exemplified 

 in the leaves of the palm, and in the fronds of 

 Laininaria. 



4. The Oblong Form. — This may be conceived as 

 derived from the fan-shape by the preponderance of 

 growth in the perpendicular radius. Such a varia- 

 tion from the semicircular shape would be . of 

 advantage where there was a deficiency of light, as 

 for example in deep water. The oblong shape, 

 unless very long in proportion to its breadth, could 

 attain no great size without becoming liable to the 

 same objection as the fan-shape — viz., offering too 

 great mechanical resistance to wind and water 

 currents. Possibly the large leaves of the Banana 

 represent the superior limit of size which is safe 

 for such a form in terrestrial plants. The shovel- 

 shaped or slab-like trees of Assyrian sculptures 

 could never have stood in a breeze of wind. 



5. The Pinnate or Feather Form. — The divisions 

 introduced into the oblong to reduce resistance give 

 rise to the pinnate form. This is a common pattern 

 among leaves, e.g. Cycads, Ferns, Umbellifers, &c. 

 Sachs observes that, speaking generally, small leaves 

 are undivided, while large leaves are mostly much 

 divided. It is reasonable therefore to suppose that 

 their divisions may to some extent be a provision 

 against injury by wind. Since the fall of the Tay 



