in ASCENT OF SAP IN STEMS 53 



weighing accurate camera lucida tracings on uniform 

 paper of sections of the wood. 



It follows that to produce the 7 cm. per hour velocity 

 observed, an actual rate of 14 cm. per hour must be main- 

 tained, while the velocity of the water through the cells 

 must be about nine times that, or 126 cm. per hour, i.e., 

 about 2 cm. per minute. But this figure has still further 

 to be increased, inasmuch as at most half of the cells' 

 lumen can at any time be active in transferring the water 

 from the lower to the upper side. The other half will, 

 by hypothesis, be occupied by the return current of the 

 protoplasm. Thus we must assume a motion across the 

 cells of at least 4 cm. per minute. This will be still an 

 under-estimate of the velocity, for it is evident that only 

 a portion of the total protoplasm can be occupied by 

 water, and also only a portion of the lumen is occupied 

 by protoplasm. Therefore, the velocity of 4 cm. per 

 minute must be regarded as an under-estimate of the 

 velocity of the current in the protoplasm required by the 

 theory. It is needless to say that such a velocity of 

 protoplasmic streaming has never been observed. Janse 

 himself states the streaming in the endodermis cells of 

 Iris, which he believes to act in the same way, amounts 

 at a temperature of 19 C. to 30(V or 0*03 cm. per minute. 

 It appears, then, that this theory which attributes the 

 lifting force of the water in the stem to the protoplasmic 

 streaming of the medullary-ray-cells requires a rate of 

 protoplasmic streaming at least 100 times that hitherto 

 observed. 



The adhesion of writers to the vital hypothesis since 

 Strasburger's results were published is so remarkable that 

 we must devote some space to examine fully the grounds 

 for their contention. 



Transmission through dead stems. When a 

 considerable length of a branch, still attached to an 

 uninjured plant, is killed by surrounding it with steam or 



