386 LECTURE XVI. 



and night do not become excessively elongated in per- 

 manent darkness. Sachs has observed, for instance, that 

 this is the case in the hypocotyls of seedlings having hypo- 

 gean cotyledons. 



An idea of the relative elongation of etiolated and of normal inter- 

 nodes will be best afforded by comparative measurements. The follow- 

 ing are from Sachs. 



Normal. Etiolated. 



Hypocotyl of Polygonum Fagopyrum 2 3 35 40 centim. 



Cucurbita Pepo 3 4 40 50 



Epicotyl Phaseolus multiflorus 32 93 millim. 



The excessive elongation of an internode is by no means 

 always accompanied by a diminished thickness. Sachs, 

 G. Kraus, and others, have observed numerous instances in 

 which the internodes of etiolated were quite as thick as those of 

 normal shoots. Kraus indeed mentions one case (Luflmus 

 termis] in which the etiolated hypocotyl was more than twice 

 as thick as a normal hypocotyl. 



We will now endeavour to ascertain the cause of the usual 

 excessive elongation of etiolated internodes, and we will 

 enquire first into the structure of these organs. It has long 

 been known that, as a rule those histological elements, such 

 as epidermal, collenchymatous, and sclerenchymatous cells, 

 which, in a normal internode, have thick walls, commonly 

 have thin walls in an etiolated internode. Further, the 

 number of the fibrovascular bundles, and the number of 

 the cells constituting them, is commonly smaller in the latter 

 than in the former. The practical importance of this differ- 

 ence in structure has been demonstrated by Koch, who has 

 shewn that the "laying" of Cereal crops is due to the 

 imperfect development of the tissues of the stem, and that 

 this is the result of an insufficient exposure to light in con- 

 sequence of the plants being too close together (see p. 137). 



The accompanying figures, due to Koch, illustrate to some extent the 

 histological differences between an internode of a plant grown in the 

 light and that of one grown in darkness. In this particular case A is 

 a transverse section of an internode of a Rye-plant grown fully exposed 



