IRRITABILITY. 389 



etc., as instances of organs which fulfil these conditions, and 

 he also brings forward, as evidence of a negative character, 

 his observation that the internodes of Cucurbita exhibit 

 normally only a very slight longitudinal tension and that 

 they do not become excessively elongated in darkness, an 

 observation which has also been made by Rauwenhoff in the 

 case of Ipomcea. But it does not appear that the longitudinal 

 tissue-tension is always small in etiolated internodes, for 

 Rauwenhoff has observed an evident tension in etiolated 

 internodes of Phaseolus, Fuchsia, Rosa, and Polygonum, and 

 Sachs mentions that he has often observed a considerable 

 tension in etiolated internodes. It would rather appear that, 

 in etiolated, as in normal internodes, the longitudinal tissue- 

 tension varies at different stages of growth, though perhaps 

 not precisely in the same way in the two cases. It is, in fact^ 

 impossible to understand the fact brought out by Rauwenhoff 

 that the fibres of etiolated internodes are not excessively 

 elongated, if the longitudinal tissue-tension remains small 

 during the whole period of growth. 



With regard to the thinness, in etiolated internodes, of the 

 walls of cells which have thick walls in normal internodes, it 

 must be pointed out that neither the different tissues, nor 

 different stems, behave alike in this respect. All observers 

 agree in stating that the epidermal and coljenchymataus cells 

 have relatively thin walls in etiolated internodes, but their 

 statements differ with regard to the fibrous and vascular cells. 

 'Kraus and Rauwenhoff have observed that the walls of these 

 cells are thin in etiolated internodes, whereas other observers, 

 notably Batalin, have not found this to be so. It may be 

 concluded that the thinness of the walls is more conspicuous 

 and constant in the case of epidermal and of collenchymatous 

 cells than in that of fibrous and of vascular cells. Kraus 

 attributes the thinness of the walls to the fact that the leaves 

 of internodes growing^ in darkness are unable to manufacture 

 the necessary plastic material for the due development of the 

 tissues. It may be admitted that if this material is not forth- 

 coming the development of the tissues will be imperfect. But 

 it must be remembered that such differences in structure 



