8o4 MECHANICS OF GROWTH. 



but also become more rigid. The considerable increase in size presupposes, how- 

 ever, from the rapidity with which it takes place, great extensibility in the cell- 

 walls. Isolated prisms of pith exposed to the air become shorter even than the 

 length they possessed in the internode^; the cell-walls which were previously in 

 a state of tension evidently contract elastically, as the turgidity diminishes from loss 

 of water. 



But if care is taken that isolated cylinders of pith do not absorb any water, 

 while at the same time they can only lose a very small quantity of it, by enclosing 

 them in a glass tube containing about i litre of dry air, they nevertheless continue to 

 lengthen perceptibly for some days, although not so considerably as when they 

 absorb water; and this lengthening affects chiefly the older parts, while the 

 youngest parts sometimes contract. The whole cyhnder 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 Senecio umbrosus 235*5 mm. long, 

 lengthened about 57 p. c. on isolation, 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 0*5 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 part i. about 18 mm. or 16*8 p. c, in part ii. about 

 23 mm. or 2i"6p. c, in part iii. about 11 mm. or 2i'6p. 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 powerfully and expands more than the 

 younger. 



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 



* Kraus, /. c, Tables, p. 29. 



