286 MEANS OF ACCELERATING TRANSPIRATION. 



and have such a form and position that the small supply of light can be utilized to 

 the full. The resultant action is just the same whether 1000 green cells are only 

 moderately illuminated, or if 500 cells are illuminated by a light twice as strong. 

 If this argument will not apply to all plants, it certainly fully applies to some, and 

 it is a fact that plants growing in damp, shady places are characterized by their 

 comparatively large, thin, delicate leaves. These leaves are also spread out 

 horizontally in such localities; they are smooth and not wrinkled; neither rolled 

 back nor bent up. Suppose we enter a thick wood in the north temperate zone, 

 perhaps in S. Germany. By the side of delicate-leaved ferns grow species of 

 Corydalis {Corydalis fahacea, solida, cava), together with species of Bentaria 

 (J), bulbifera, digitata, enneaphyllos), dog's mercury {Mercurialis perennis), Isopy- 

 rum thalictroides, bitter vetch {Orobus vernus), woodruff (Asperula odorata), 

 Lunaria rediviva, herb Paris (Paris quadrifolia), cuckoo-pint (Arum maculatum), 

 spurge-laurel (Daphne Mezereum), and many other species belonging to very 

 different families, but all having the common characteristic of possessing flattened 

 leaves and no covering of hairs. If a brook ripples through the shady wood, 

 growing on its banks will be found the yellow balsam (Impatiens nolitangere), 

 the broad-leaved garlic (Allium ursinwrn), Streptopus amplexifolius, and the 

 butter-burr (Petasites officinalis), with its huge foliage, all again characterized by 

 their large, smooth, flat leaves. In such places in S. Germany are generally to be 

 found the largest leaves. Those of the butter-burr attain to a length of over a metre, 

 and are almost a metre broad. The fronds of the common bracken-fern (Pteris 

 aquilina) are equally large in such situations; and on the ground in damp, shady 

 alder woods, growing in comparatively cold mountain glens, another fern (Poly- 

 podium alpestre) is to be met with, whose frond is IJ metres long. But they only 

 possess these extended leaves when growing in the situations described, in the damp 

 air of cool and shady woods. One would expect that under similar conditions 

 outside the wood, the leaves would exhibit a more luxuriant growth, and would 

 attain to a still larger size in consequence of the influence of a higher temperature; 

 but this is not the case. In the drier air and sunshine on the unshaded banks of a 

 rivulet, the leaves of the butter-burr are scarcely half as large as those growing in 

 the neighbouring cold shady glen, from whose dim light the brooklet flows out into 

 the open country; and on sunny ground neither of the two above-named ferns will 

 even approximately reach that size to which they grow when surrounded by the 

 cold, damp air in the depth of the alder wood. 



This difference in the relative size of the leaves of one and the same species^ 

 according as to whether they grow in sunny places in dry air, or in shady spots in 

 damp air, is sometimes carried so far that the whole physiognomy of the plants 

 becomes altered, and they might easily be thought to belong to distinct species. 

 Thus plants of Convallaria Polygonatum, growing in shady meadows watered 

 by rivulets, show leaves at least three times as large as those which grow on the 

 rich damp earth on the steep sides of rocks down which water rushes, where they 

 are warmed by the sun all the day. This comparison might be illustrated by 



