536 LECTURE XXXI. 



contrary, require the light in a high degree in order to grow normally. This fact is 

 so much the more remarkable since very many roots and subterranean shoots 

 exhibit no signs of such sensitiveness ^ 



While the lower segments of the potato-shoot afford us an example of the fact 

 that under some conditions the growth of parts of the plant is almost entirely 

 prevented by the light, an example of the contrary is afforded, according to Goebel's 

 observations, by the scale-leaves of some rhizomes, which normally grow in subter- 

 ranean darkness. These if exposed artificially to the light have their growth and 

 all the processes of configuration promoted, and become transformed into true 

 foliage-leaves. Goebel ^ cultivated plants of species of Circcea so that the otherwise 

 subterranean stolons were forced to develope further in the light : their otherwise 

 scale -like leaves now became transformed into normal green foliage leaves. 



The fact established by Stahl that the assimilating parenchyma of the foliage- 

 leaves undergoes conspicuous changes in the form of its cells according to the 

 intensity of the light, definite relations between the directions of growth and the 

 direction of the ray of light being apparent at the same time, also belongs to the effects 

 of light on post-embryonic growth. As is shown in Fig. 202 (p. 247) the assimilat- 

 ing tissue of the usually thin flat leaves forms two layers; an upper one turned 

 towards the light consists of palisade-like upright cells elongated in the direction of 

 the incident rays of light ; the lower half of the assimilatory parenchyma is composed 

 of cells which are rounded or often transversely elongated in a plane parallel to the sur- 

 face of the leaf, and between which there are usually large intercellular spaces. These 

 layers are distinguished as the palisade layer and the spongy layer of the mesophyll. 

 In leaves with vertical surfaces, äs the Iris, and in stems like those of Equisetum, 

 the palisade cells on the other hand lie horizontally, in the former on both sides of the 

 leaf, in the latter radially directed on the whole towards the periphery. According to 

 Stahl the palisade cells, however, constitute that form of assimilating tissue which 

 is especially produced by intense light striking the surface of the leaf directly, 

 since he shows that in the same species of plant the formation of palisade tissue 

 diminishes in proportion as the intensity of light decreases. Leaves grown in 

 the shade possess chiefly or only spongy parenchyma ; those grown in strong light 

 chiefly palisade tissue. As an example the Beech may be mentioned, as well as 

 other trees. We have here in view, in the first place, only the fact that light is the 

 cause of these relations of organisation ; but it must be added that the histological 

 differences thus produced are in their tiirn again of use for the process of assimilation, 

 a matter however into which we cannot here proceed further. 



Our considerations so far have been concerned with the action of Gravitation and 

 of Light on growth. The former is always acting continuously on each of the minutest 

 parts of the plant : light, however, only acts on those parts which are above ground. 



^ The action of light on young Potato-shoots described above is the more remarkable when the 

 nights are taken into consideration : it might be supposed that although the growth is completely 

 arrested in the day-time, it could nevertheless proceed in the night, and perhaps the extremely slight 

 growth actually takes place only during the nights. But the growth is so slight that even on 

 this assumption it seems probable that an after-effect of daylight persists even during the night. 



^ Goebel on Ciiraa, in his 'Beiträge zur Morphologie und Physiologie des Blattes'' (Bot. Zeitg. 

 j88o). 



