PHENOMENA DUE TO THE TENSION OF TISSUES. 725 



youngest parts sometimes contract. The whole cylinder becomes dry and rigid on 

 the surface. Out of a large number of observations the following may be chosen to 

 elucidate this point. 



A prism of pith from a part of a shoot of Sciiecio umhrosus 235*5 mm. long, 

 lengthened about 57 p. c. when isolated, and weighed 5-3 grammes. It was divided 

 into three parts by marks of Indian ink; their lengths being: — i. (the oldest) 100 mm., 

 ii. 100 mm., iii. (the youngest piece) 49 mm. The prism of pith was now fixed in 

 a dry glass tube, which was then corked at both ends. After fourteen hours the 

 parts had lengthened as follows : — part i. about 4-5 mm., part ii. about 6-5 mm., part 

 iii. about 2 mm. or 4*1 p.c, while the pith had lost 0*15 grm. of water. After re- 

 maining for twenty-six hours more in the glass tube the following further changes 

 had taken place ; part i. had again lengthened about 2*5 mm., part ii. about 0*5 mm., 

 while the length of part iii. had diminished about 05 mm. No further loss of 

 water had taken place, because the glass tube had become covered with moisture. 

 The pith was now placed in water, and after six hours the following increase of 

 length had taken place:— in parti, about 18 mm. or i6-8 p.c, in part ii. about 

 23 mm. or 21-6 p. c, in part iii. about 11 mm. or 2i'6 p.c. (as compared with the 

 length before placing in water). The pith had also become considerably thicker, 

 having absorbed 6 grammes of water. The estimation of the dry weight showed that 

 the pith contained only 0*22 grm. of solid substance ; this was combined, when the 

 pith was isolated, with 5*08 grm. of water ; it subsequently lost 0*15 grm., but by the 

 end of the experiment had again absorbed 6 grm. At first therefore the pith con- 

 tained 4*23 p.c, at last only 1*97 p.c. of solid substance. Experiments of this 

 kind show that the pith of the youngest internodes loses its water most easily by 

 evaporation, as is shown by its decrease in length. Kraus was led by other ex- 

 periments to the same conclusions ; and he also showed — not in contradiction, as 

 he thought, but in harmony with these results {I.e. p. 123) — that the older pith of 

 growing internodes attracts water more eagerly and expands more than that of 

 younger internodes. 



If the question is now asked how the lengthening of the pith can take place in 

 spite of the loss of water (though this may be small), it must first of all be noted that 

 its surface becomes remarkably dry under the circumstances described. It is scarcely 

 possible to attribute this significant desiccation of the surface to the small loss of 

 water indicated by the weight ; it is probably rather caused by the inner cells of the 

 pith withdrawing water from the outer cells, and thus lengthening; but the outer 

 cells would become shorter if they were not stretched by the inner ones. That this 

 is actually the case is shown by the rigidity of the pith under these circumstances, 

 caused by the tension that subsists between the dry outer layer and the moister 

 inner mass. If the prism of pith is divided lengthwise, the parts curve outwards; 

 and sometimes the outer surface becomes even strongly concave. If the inner cells 

 of the pith are able to withdraw water from the outer ones, it may be inferred that 

 the outer cells are also able to withdraw it from the surrounding wood and especially 

 from the peripheral tissues, preventing these from becoming strongly turgid ; their 

 growth being thus retarded in favour of that of the pith, by which they are now 

 placed in a state of passive tension. It is noteworthy that the medullary cells which 

 contain a minimum quantity of dissolved substances nevertheless absorb water 



