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 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 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. 1 00 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 o'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 i6-8p. c, in part ii. about 
23 mm. or 21-6 p.c., in part iii. about 11 mm. or 21-6 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 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. 
