IRRITABILITY. 385 



shoots which have grown from tubers, the one under normal 

 conditions, the other in darkness. We are first struck with 

 the difference in colour between the two shoots: the one 

 which has grown in darkness, the etiolated shoot, has a 

 white stem, and leaves which are at first pinkish, owing to 

 the presence of colouring-matters in the sap of the cells, and 

 subsequently pale yellow, whilst the other, the normal shoot, 

 is green, leaves and stem alike. The former is destitute of 

 chlorophyll, the latter possesses it (p. 262). The next point 

 of difference is in the length and thickness of the internodes 

 of the stems, those of the etiolated plant being much longer 

 and more slender than those of the normal plant. Further, 

 the angle made with the main stem by the lateral branches 

 and by the petioles of the leaves with the main stem is 

 smaller in the etiolated than in the normal shoot. Finally, 

 the smallness of the leaves of the etiolated shoot, as com- 

 pared with those of the normal shoot, attracts our attention, 

 but, as we have already fully discussed that subject, we will 

 now confine our attention to the internodes. 



Shoots, then, differ from most leaves in that continuous 

 darkness does not arrest their growth. But excessive elon- 

 gation of the internodes is not always exhibited by shoots 

 which have grown in permanent darkness. It is exhibited 

 by the majority of shoots which are adapted for growth 

 in length under the normal alternation of day and night. 

 Some shoots of this kind seem, however, as Sachs has pointed 

 out, to attain their maximum of elongation under normal 

 conditions, and these do not become excessively elongated 

 when they grow in permanent darkness. As instances of 

 such " normally etiolated " shoots, as he calls them, Sachs 

 mentions those of Dioscorea Batatas and of the Hop. Other 

 shoots, again, which have no natural tendency to elongate 

 when growing under normal conditions, do not do so in 

 darkness. This was found by Sachs to be the case in 

 the Beetroot and in Cactus speciosus ; the etiolated shoots 

 of the latter plant had in fact shorter internodes than 

 those of normal shoots. Shoots which are not adapted 

 for growth in length under the normal alternation of day 

 V. 25 



