272 COMPENDIUM OF GENERAL BOTANY. 



Let us consider the behavior of the potato-plant, according to 

 the observations of Sachs.' The 3'oung shoots (the " eyes ") of 

 the tuber require darkness for their growth ; light hinders their 

 development in a remarkable degree ; later the different mem- 

 bers and leaf-organs developing from them must have sunlight 

 for their normal development. The shoots first named are 

 adapted to grow in darkness ; from them the tubers are devel- 

 oped. Many other subterranean organs do not show this sen- 

 sibility to light. Let us consider two simple experiments. 1. 

 Observe the growth of seedlings and shoots in permanent dark- 

 ness. 2. Place one part of a plant in the dark, while the remain- 

 ing parts are normally illumined (according to the experiment by 

 Sachs). In these cases growth in the dark can take place only at 

 the expense of the reserve-materials or of food-substances formed 

 in adjacent tissues under normal surroundings. In case the 

 seedlings and shoots show a rapid growth of the axial organs 

 they become abnormall}' elongated, with usually slight develop- 

 ment of the mechanical cells. The leaves remain dwarfed, but 

 this is not directly due to the lack of sunlight, for case 2 shows 

 that leaves may develop on the darkened plant portion, though 

 they may not be of normal size. The darkened portion (2) 

 may even develop normally colored flowers and fruit. In both 

 cases and in general it may be stated, that leaves developed in 

 the dark are devoid of chlorophyll ; they are said to be " chlorotic," 

 or etiolated.^ 



From the study of these pathological changes and the peculiar 

 differences associated therewith we are enabled to understand 

 their teleological significance. Cotyledons which normally re- 

 main under ground, and therefore do not become very much 

 elongated in the hypocotyledonous stem-portion, do not show 

 any abnormal elongation in permanent darkness. On the other 

 hand seedlings whose cotyledons are normally raised above the 

 soil show the described elongation wdieu growing in the dark. 

 Again, not all foliage-leaves remain small in the dark ; many 

 blade-like monocotyledonous leaves become abnormally long 

 and slender in the dark. This is also true of the leaves of the 

 onion ; by their elongation they are enabled to rise above the 



' Vorlesnngen, p. 650. 



' Sachs, Abhandlungen uber Pflanzen-Phys. p. 194. (1893.) 



