194 LECTURES ON BIOLOGY 



regarded as the leaves of the cactus. However, the leaves 

 proper of Opuntia have degenerated into spines. If we keep 

 this plant for some time in a cellar or in a darkened room the 

 stems continue their growth cylindrically, i.e., in the original 

 form, and of a flattening there is no longer a trace. Again, in 

 many orchids it is the roots which carry the chlorophyll and per- 

 form the functions of leaves. As long as the roots remain hidden 

 in the moss-cushions of the branches of the trees on which these 

 orchids are parasitic they retain their cylindrical form. Only 

 when they come forth into the light, i.e., when they can take 

 up their new functions of assimilating carbon dioxide, they 

 proceed to flatten. 



A still better proof is supplied by one of the liverworts, 

 Marckantia, which flourishes in many districts in large colonies 

 in moist sandpits, on stones, or in flower-pots. This little plant 

 possesses a rather large dorso-ventrally flattened thallus, the 

 upper side of which is strikingly differentiated in structure from 

 the under side. Nevertheless it is uncertain, in the spores as 

 well as in the germ-discs originating from the spores, which of 

 the parts will afterwards develop into the upper side and which 

 into the under side. But if we expose the germ to daylight only 

 for two or three days it becomes definitely settled, though 

 neither a flattening nor differentiation is visible, that the illu- 

 minated side is to be the upper side. Whatever we may now 

 do with the young plants will not alter this fact : a stimulus 

 extending only over a short time was sufficient permanently 

 to fix its structure. 



Different is the behaviour of the prothallus of the ferns. In 

 his the non-illuminated underside which lies on the ground carries 

 the rhizomes and the male and female sex-organs. But if we 

 make a newly-formed prothallus pass its development swimming 

 on a nutrient medium we may, at will, change the upper 

 into the under side, and back again into the upper side, merely by 

 altering the lights. In order to fix the structure definitely it is 

 in this case necessary to let the stimulus remain active through- 

 out the entire period of development. 



If I now mention that the ivy forms its aerial roots invariably 

 on the shady side, that the anatomical structure of the leaves of 

 foliage trees differs in proportion as they are well or indifferently 



