THE USES AND ORIGIN OF THE ARRANGEMENTS OF LEAVES IN PLATO 409 



special significance (if any ever existed), and when they could have stood in 

 more immediate and important relations to the conditions of the plant's exist- 

 ence. In this inquiry our principal guide must be hypothesis, but it will be 

 hypothesis under the check and control of the theory of adaptation. It will 

 not be legitimate to assume any unknown form as a past form of life, and as 

 a basis for these arrangements, without showing that such an hypothetical 

 form would have been a useful modification of a still simpler one, which still 

 exists and is known. In this way we may be able to bridge over the chasm 

 that separates the higher and lower forms of vegetable life. 



Our problem then becomes, Whether, in the absence of any surviving 

 instances of continuous spiral leaf-like expansions on vegetable stems, we can 

 find any utility in such an arrangement that could act to modify simpler 

 known forms, and convert them into this? If we suppose our hypothetical 

 spiral leaf-blade to be untwisted, it becomes a single-bladed frond, or a frond 



with one of its blades undeveloped. In considering what advantage there could 



**~.„.»v« 6 



be in the twist, we should revert to the general objects or functions of leaf- 

 like expansions*. They are obviously to expose a large surface to the action 

 of light on its tissues, and to bring it into the most complete contact with 

 the medium in which the plant lives, — with water, or, in more advanced 

 ants, with the air. Secondly, to accomplish this with the least expenditure 

 of material; not by an absolute, but a relative economy, which has reference 

 to the needs of other parts, like the stem or the roots. In many of the 

 higher plants the developments of the stem serve to diminish to the utmost 

 the amount of this material, and the needed expansion, by giving to them 

 advantageous positions. The first of these objects is secured in the simplest 

 and rudest manner in the algce, as represented by the seaweeds. This is a 

 simple expansion of cellular tissue. But even here we do not find perfectly 

 plane surfaces, facing only two ways, and allowing the water to glide smoothly 

 and unobstructed over them. The corrugated surfaces of many of them, and 

 in the large leaves of some land-plants, are doubtless due to unequal growths 

 in the cellular tissues ; but such a physiological explanation of this feature 

 does not preclude the supposition of its being a fixed character in a plant, 

 or becoming such in consequence of its utility. It certainly serves the purpose 

 of opposing the leaf-surface to many directions, both with reference to the 

 incidence of light, and to the movement of the surrounding medium, — to 

 water-currents, or to breezes. Segmentation, again, such as is seen in the 



