68 THE FLOWERING PLANT. 



of its constructive function. A plant placed in darkness soon 

 gets pale and unhealthy, and though it may grow considerably in 

 length, gradually becomes less in weight, ultimately dying. Such 

 a plant is said to be etiolated. Celery, and some other garden 

 plants, are purposely reduced to this condition by heaping earth 

 around them. They thus become very sickly, and the tissues 

 not being vigorously developed, are very tender. Also, in the 

 case of celery, the characteristic essential oil is too strong and 

 too abundant under normal conditions to let the shoots be eaten. 

 Chlorophyll cannot be formed, in most cases, unless light is pre- 

 sent. If a bean-seed is germinated in the dark, a yellowish 

 etiolated seedling is produced, which grows to a certain extent 

 at the expense of the reserve materials stored up in the seed. 

 The yellow colour is due to the development of etiolin, which is 

 closely related to chlorophyll, and exists, like it, diffused through- 

 out the substance of protoplasmic granules (cf. p. 8). It is 

 also to be noticed that the seedling weighs less than the seed 

 from which it grew. A very feeble light suffices to convert 

 the etiolin into chlorophyll, so that the granules in which it 

 occurs become chlorophyll granules. If the now green seedling 

 is placed in a fairly strong light, it will rapidly increase in size 

 and weight, since its chlorophyll is able to build up organic com- 

 pounds. The roots of the seedling must of course be placed in a 

 suitable soil or food solution, but this alone is useless if light is 

 excluded. 



Since carbon dioxide is one principal item of food, it is obvious 

 that chlorophyll cannot work unless supplied with it. This is 

 readily proved by growing plants under a bell- jar to which air is 

 freely admitted which has been deprived of its carbon dioxide by 

 means of caustic potash. Under these circumstances the plant 

 does not increase in weight, and starch cannot be detected in its 

 leaves. The palisade parenchyma is the most important part of 

 the leaf for the purposes of assimilation, and therefore, as we 

 have seen, is developed on the side turned towards the light. 

 Where, as in vertical leaves, the conditions of illumination are 

 equalized, there may be such tissue on both sides. It is also 

 interesting to notice that, in plants with dense foliage, such as 

 trees, the leaves most exposed to light are thicker than the 

 others, owing to the formation of extra palisade layers. 



As already explained (p. 10), the assimilatory process carried 

 on by chlorophyll involves the liberation of a large quantity of 

 oxygen, which passes off into the surrounding medium. This is 

 easily proved by experiments, one of which consists in cutting off 

 a vigorous shoot from a water plant, and, by means of a small 

 weight, keeping this submerged, cut end up, in a vessel of fresh 



