534 THE AMERICAN NATURALIST. (VoL. XXXIX. 
as compared with the Coniferz, but that it is of .greater volume 
and requires a correspondingly more capacious channel; while 
such channels must also be direct and involve fewer impediments 
to free circulation in a longitudinal direction. These require- 
ments are met by the large, transverse volume of the individual 
vessel; by the great multiplication of vessels so characteristic of 
Catalpa, Salix, Populus, etc. ; by their repetition in the secondary 
xylem of each year's growth, and in the provision for very free 
longitudinal movement of fiuids as expressed in the sieve-like 
terminal walls, or in the complete obliteration of such interposed 
septa. Although such vessels retain their capacity for the con- 
veyance of the nutrient fluids for a long time, 7. e., several years, 
they eventually become functionless, often through the forma- 
tion of extensive thyloses as in the Catalpas, members of the 
Salicacee, Quercus, etc., and in general terms the functional 
capacity of the vessels cannot be restored when so lost, while 
under certain circumstances, the development of thyloses may 
assume a definitely pathological aspect (Watt, :01). From the 
observations thus far made, it is evident that the particular evo- 
lution of the protoxylem which results in the formation of such 
vessels, provides elements which are solely concerned in the 
movement of the transpiration current, and which have, as it 
were, appropriated this function in such an exclusive sense as to 
eliminate it from all other structural elements of the xylem, so 
that these latter are thereby left free to develop in the most 
responsive way under the special influence of other requirements. 
We may now ascertain the special features which serve to dis- 
tinguish the second course of evolution from the protoxylem, 
already referred to. The excessive development of the conduc- 
tive tissue tends to diminish the general strength and call for 
the formation of purely mechanical elements to an extent other- 
wise unnecessary. This factor operates, therefore, to emphasize 
the divergence of the two lines of structural evolution, giving 
greater prominence to the conductive elements on the one hand 
as already shown, and on the other hand to the mechanical ele- 
ments. Certain of the protoxylem elements, otherwise potential 
vessels, are diverted in growth, and instead of expanding, they 
contract with a correspondingly greater development of the sec- 
ondary wall, and assume a strictly fibrous form. This necessa- 
