BEHAVIOUR OF THE PITH. 57:} 



The prism of pith was placed in a wide glass tube, which was then corked 

 at both ends. In the course of fourteen hours the following elongations had 

 occurred : — 



The part I had elongated 4-5 mm. 

 „ II „ „ 6.5 „ 



„ III „ „ 2-0 „ (i.e. 4-i7o)- 



The pith had however meanwhile lost 0-15 gr. water. 



After another twenty-six hours stay in the glass tube the following fresh changes 

 had made their appearance : — 



The part I was still longer by 2-5 mm. 

 » II „ „ 0-5 „ 



„ III was shorter by 0-5 „ 



During this interval no further loss in weight had occurred, because the air in the 

 glass tube was now saturated with aqueous vapour, and thus no more evaporation 

 could take place. 



The pith was then laid in water, and in six hours the following changes had 

 occurred : — 



The part I was longer by 1 8 mm. 



„ II „ „ 23 „ 

 „ III „ „II „ 

 The pith had at the same time become considerably thicker, and had absorbed 6 gr. 

 water; though at first, as said, it had only weighed 5-3 gr. 



The determination of the dry weight showed that there was in this state only 

 0-2 2 gr. of solid substance ; this substance after the isolation of the pith was combined 

 with 5-08 gr. water, and then lost 0-15 gr. At the end of the experiment, however, 

 it took up 6 gr. more ; or, at first the pith contained 4-23 °/^, at the end only 1-97 °/o 

 of sohd substance. 



It is seen from these statements, as I have already pointed out, that in the cells of 

 the pith at any rate only an extremely dilute solution capable of causing endosmose 

 can be present, and that this nevertheless causes a very pronounced absorption of 

 water, turgescence, and growth. There is one other point in our experiment to be 

 explained, however, namely, the original elongation of the pith in spite of a loss 

 (though small) of water. The explanation lies, however, in the very pronounced drying 

 up of the surface, which could not well be due only to the small loss of water by evapo- 

 ration ; it is on the contrary probable that the inner cells of the pith took the water 

 from the outer ones and thus elongated, the external desiccated ones being passively 

 extended just as the epidermis is elsewhere. That this was actually the case is shown 

 by the stiffness of the pith in other experiments of this kind ; if the prism of pith which 

 has become dry on its outside is bisected longitudinally, the parts gape outwards, as 

 when a living shoot-axis is split longitudinally. I drew from these facts the following 

 conclusion. If the inner pith cells are able to abstract water from the outer ones, it 

 may be assumed that the external cells of the pith also are able to abstract water 

 from the tissues surrounding them, and therefore retard their turgescence, whence 

 their growth is retarded, and the further consequence is that they become passively 

 extended by the pith, which robs them of water. 



