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begins with the Hepatics and ends with the Composite. He 
gives the two theories regarding the function of these threads, 
first, that they serve as conductors of dynamic stimulus, second, 
as a means for conducting the various food elements to the differ- 
ent parts of the plant. 
According to the first, the sieve-tubes and their contents cor- 
respond to the nerves of the animal. He gives the opinions of 
Pfeffer, Haberlandt, and many others in favor of this theory, and 
quotes from Pfeffer that even the cell wall may contribute to this 
effect by the motion of its particles uniting with that of the pro- 
toplasma molecules. All these authors agree, in general, that 
this conduction of stimulus is one function of the continuous pro- 
toplasmic threads. There is, however, much less unanimity in 
regard to the second theory. Various objections are quoted ; 
among others, the exceeding fineness of the perforations in the 
* wall through which the threads pass. The author thinks this 
objection of little value, even if it were not possible to show in 
many cases that the perforations were of considerable size. 
He says that if we discard this theory, which, in general, has 
reference to the passage of organic material, we must suppose 
this material finds its way through much finer channels, namely, 
between the molecular interstices of the wall, either by filtration 
or osmosis. Here is given the experiment of De Vries to show 
that by diffusion alone, even those substances conveyed the most 
rapidly, e. g., cane-sugar and salt, would find too slow trans- 
port to meet the needs of the plant. According to this experi- 
ment it would take a milligram of salt in a ten per cent. solution 
three hundred and nineteen days to move the distance of a meter 
in water. Albuminous substances would require fourteen years. 
By comparing the results obtained by his own studies with 
those of many others, the author decides that an important func- 
tion of these threads is the transport of organic matter to various 
parts of the plant. Furthermore, that they may serve as conduc- 
tors of stimulus, as by their means the whole protoplasmic con- 
tents may be withdrawn from one part of the plant to another. 
This latter view, he says, may account for the entire lack of 
_ protoplasm in the ducts and schlerenchymatic cells whose walls 
are completed in thickness. As soon as this thickening process 
