No. 467] STUDY OF THE SALICACEE. 805 
difficult not to feel that we have before us an expression of 
evolution in two directions from the protoxylem, the one lead- 
ing directly to the formation of vessels with multiseriate 
bordered pits; the other leading to the formation of purely 
mechanical and fibrous elements wholly devoid of pits, or in 
which the latter survive in the form of simple, sporadic per- 
forations of small size. As far as it is possible to interpret 
the alterations of structure so far observed, the following would 
seem to be the sequence of events : — 
Protoxylem. 
Spiral tracheids with close spirals set at a low angle and showing a bor- 
dered pit structure in section. Terminations usually abrupt. 
Scalariform tracheids with abrupt or tapering extremities ; the markings 
showing a bordered pit structure in section. Transitional forms numerous. 
Markings often obliterated from either the radial or the tangential walls. 
Secondary Xylem. 
The elements diminish in breadth 
and increase in length. 
Bordered pits are at first present 
on both the radial and the tangential 
The elements diminish in length 
and increase in breadth. 
The bordered pits on both the 
radial and the tangential walls are 
successively I-, 2-, and multiseriate. 
The vessels thus formed are mul- 
tiplied in large numbers ; they serve 
the purpose of circulation ; they are 
reproduced as features of the sec- 
ondary wood of each year's growth ; 
they do not form a definite zone or 
medullary sheath after the first 
year; their functional activity may 
be arrested by the excessive develop- 
ment of thyloses or other causes; 
they constitute a relatively small 
portion of the entire structure ; they 
contribute to the greater porosity 
and diminished strength of the struc- 
ture as a whole. : 
walls; at first one-seriate, they di- 
minish in number and become more 
strictly segregated; they become 
more strictly confined to the radial 
stress, to which end they are especi- 
ally adapted ; they constitute the 
stem, to which they impart hardness, 
solidity, rigidity, and durability. 
