300 LECTURE XVIII. 



The uninitiated student easily draws thence the erroneous conclusion that the growing 

 plants must also be nourishing themselves. In the following lecture I shall show 

 more in detail that growth of the organs and assimilation in the chlorophyll are two 

 processes mutually independent in a. high degree. In order that a plant, or certain 

 of its parts, may grow, it suffices that organic matters assimilated by the previous 

 activity of chlorophyll are present; and, conversely, a plant may be assimilating 

 vigorously without growing at the same time. When, therefore, plants grow in the 

 dark, or with insufficient illumination in general, this takes place at the cost of such 

 assimilated substances as have previously been stored up in the tissues of the seeds, 

 tubers, bulbs, root-stocks, or in the cortex of the branches of trees, etc., and which 

 are now utilised in growth. Moreover, growth with insufficient illumination destroys 

 considerable quantities of organic plant-substance, in consequence of the respiration 

 connected with it, as I shall demonstrate in detail in a later lecture. Hence it comes 

 about that the dry weight of plants growing in the dark is diminished, and this 

 growth in the dark can therefore only continue until the existing reserve-materials 

 have been used up. 



For the sake of completeness, this opportunity may be taken of incidentally 

 touching upon another point. As the instrument of assimilation can only have 

 its assimilating activity set at work under the influence of light of sufficient intensity, 

 so also it is in most plants only brought to complete development by the light. 

 The leaves of flowering plants, especially when they are formed by growth in the 

 dark, produce chlorophyll-grains in their cells, it is true, but these are not 

 green ; they remain yellow ^, tinged with a colouring matter which is but little 

 different from the green colouring matter, but which is unable to communicate 

 to the chlorophyll-grain the power of decomposing carbon dioxide. Such yellow 

 etiolated leaves do not assimilate, as experiment shows ; exposed for some time to 

 light, however, even though feeble, they become green, and are then able to 

 decompose carbon dioxide. Nevertheless, as I established twenty-three years ago, 

 the development of green chlorophyll is not in every case connected with the 

 influence of light : the primary leaves of the seedlings of Conifers develope their 

 -normal chlorophyll even in profound darkness, and the leaves of Ferns behave 

 similarly. This dependence of the formation of chlorophyll upon light (not, as we 

 see, without exception) often led formerly to the erroneous assumption that the 

 greening of leaves is itself a process of assimilation, and connected with the evolution 

 of oxygen. A direct refutation of this view, however, lies in the observation that 

 very feeble light enables the yellow leaves produced in the dark to become green, 

 although this feeble light is far from sufficient to cause the green chlorophyll-grains 

 to assimilate. This important fact, much too little noticed, may be very easily 

 confirmed by allowing seedlings of Beans, Maize, Tropaolum, etc., to germinate 

 in pots situated at the back of an ordinary dwelling-room. Their first leaves 

 become green: that no assimilation follows, however, is shown by weighing the 

 dried plants, which yield a smaller weight of organic substance than was contained 



' On the behaviour of chlorophyll in the dark, cf. my ' Experimental- Physiologie' p. 317, and 

 Pfeffer, ' P/lanzen-Physiologie,' p. 221.— On the dependence of the formation of chlorophyll upon 

 temperature, see Sa.c!as, Flora, 1864, p, 497, and 1862, p. 129. 



