Senecio prcBcox, D. C, from Mexico. 



37 



C CR 



partitions of pith, have increased to such an extent as to form 

 chambers one above another throughout the stem (Figs. 2 a 

 and 2 b). Water is again absorbed, if the stem is immersed in 

 that liquid, as I proved after the dry stem had been cut, so that 

 the pith again swells up and assumes a nearly normal aspect. 

 One remarkable exception, however, was found to the general 

 dry appearance above described. At the very base of the stem, 

 right at the cut off extremity, where one would least expect to 

 find it, separated by two membrane-like diaphragms cd from 

 the end LC, was a single, watery, turgescent disc td, as shown in 

 the accompanying 

 figure. The rea- 

 son for the pres- 

 ence of the single 

 turgid disc was 

 not hard to find. 

 The proper con- 

 duction of the 

 water from the 

 disc as from the 

 other discs was 

 prevented by the 

 injury which the 



adjoining wood cells sustained, as indicated by their some- 

 what dry and brown appearance (Text Fig. is). 



From a long series of careful and painstaking experiments 

 beginning with those of Hales in 1727, we know that water 

 and other crude substances travel up through the alburnum of 

 stems. The elaborated materials on the other hand are car- 

 ried down through the phloem, or bast portion of the stem. 

 In Senecio prcBcox during the sixteen months that elapsed after 

 the piece of stem was collected, the water was gradually baled 

 out of the pith discs, or reservoirs, and was carried to the grow- 

 ing point (Plate VIII, Figs, i and 2). The leaves in ordinary 

 green plants lose daily large quantities of water by transpira- 



