CHLOROPHYLL AND LIGHT INTENSITY. 391 



How quickly a glaring light is able to destroy the chlorophyll can be well seen 

 in the green Sea-lettuce on the shore below. In a high sea a violent wave tears 

 fragments of the Ulvaceae, known under the name of Sea-lettuce, from the coast- 

 rocks ; a second wave as it rushes up washes the leaf -like structures on to the 

 shingle of the shore, and there it remains with other debris lying amongst the 

 stones. The sea now becomes calm, the sky has cleared, the sun's rays are again 

 burning with undiminished strength on the shadeless strand. As long as the Sea- 

 lettuce adhered closely to the rocks below the surface of the water it displayed a 

 brilliant emerald green; the water in which it was submerged to some little depth, 

 even at a low tide, sufficed to somewhat temper the sunlight; but the stranded Ulva 

 is deprived of this light-regulating covering of water, and in a few houi'S its 

 chlorophyll is destroyed. It is turned yellow, and looks like a lettuce-leaf which 

 has lain for a week in a dark cellar. A similar appearance is also seen in confervas 

 and spirogyras which fill stagnant pools of water with their masses of united fila- 

 ments. Two decimetres below the water they display a beautiful dark-green colour, 

 while close to the surface they appear a yellowish-green, and if the pool dries up so 

 that the masses of filament come to lie on the damp slime, in two days they are 

 quite bleached; the undimmed sunlight has completely destroyed the chlorophyll in 

 the cells. In the depth of beech- groves the Woodruff (Asperula odorata) raises its 

 leaves arranged in whorls on the stem ; over it the thickly-leaved branches of the 

 beeches bend together, forming a roof through whose interstices only here and there 

 a weak sunbeam finds its way into the depths. In the dim light the leaf -stars of 

 the Woodruff appear of a deep, dark -green tint. Now the axe of the woodcutter 

 resounds through the forest — the beeches are felled, the shading roof of foliage is 

 demolished, and the floor of the wood is exposed to the glaring sunbeams. Within 

 two weeks the Woodruff can no longer be recognized; it has become sickly and pale; 

 the leaf -stars have lost their dark green, and the chlorophyll has been destroyed by 

 the glaring light. The same thing occurs with ferns as with the Woodruff. In the 

 dimness of the floor of the forest, between steep-walled rocks, and on shady northern 

 declivities they are tinted dark green; in sunny situations they become pale, 

 and then are noticeably retarded in growth. All these plants are not organized to 

 adapt themselves, in the case of an alteration of the illumination of their habitat, to 

 the new conditions and to protect themselves from the undimmed rays falling on 

 them. They are only fitted for the shady floor of the wood, and an over-abundance 

 of hght is their death. 



But how is the vegetation protected in a habitat where during the whole of 

 the vegetative period full light predominates, where the sun makes itself felt from 

 rise to setting with uninterrupted power? It has already been pointed out that the 

 plants on the broad ridges and terraces of the rocky shores of the Mediterranean are 

 shrouded in dull grey, clothed in silk or wool, or else overstrewn with chaff-like 

 scales, and consequently have lost their fresh green colour. In reality it is not 

 quite correct to say that they have " lost " the green, for their parenchymatous cells, 

 especially those of the palisade and spongy tissues in the foliage-leaves, are no less 



