202 
the radial walls, while those of the autumn wood are found 
almost exclusively on the tangential walls. It has been sug- 
gested that this arrangement is just what might be expected if 
these pores are designed to facilitate the passage of water. In 
the early spring, when the building process begins anew, there 
arises at the same time the necessity for a sufficient supply of 
water for the new cells, which necessity is met by these pores 
forming a direct communication with the cambium layer. This 
in case of the conifers is given only as a suggestion or conjec- 
ture; but whatever importance may be attached to it as such, 
may be applied with equal weight to the wood of Quercus, and 
also to that of the remaining genera of this and the other seven 
families. In these eight families where both bordered and simple- 
pored libriform tissue occurs, of which Quercus is given as the 
type, we find the last layers. of the summer’s growth, the so- 
called autumn-wood cells, contain bordered pores, nearly all of 
which are situated on the tangential walls, thus furnishing the 
same means for the rapid transport of water to the cambium 
layer in the spring as in case of the conifers. The first cells 
built in the spring are the large trachez and thin- walled tracheids, 
which arrangement appears also a part of the same plan, as the 
rapid growth occurring when the leaf-buds develop into leaves, 
must call fora large and rapid supply of water. Later in the 
season are built the strengthening cells, real libriform with simple 
pores, with no special provision for water transport; then in the 
fall again appear the pores with large borders. 
In Castanea and Fagus the arrangement is the same as in Quer- 
cus, except that in Fagus the zone of simple-pored libriform tissue 
is very harrow. In the genera examined in Amygdalee, this 
zone is much wider than in any of the above mentioned cases, 
while in Spzr@a the case is reversed, the greater part of the 
year’s ring being made up of bordered-pored libriform. In the 
remaining families, Celastrineze, Sapotaceze, Campanulacez, and 
in Buxus, it is more difficult to determine the average amount of 
- simple-pored libriform. In Celastrineze and Buxus the greater 
part of the libriform tissue contains bordered pores. The family 
Myrtaceae we have also included in this number, as it contains 
both kinds of libriform tissue in all the genera examined, with 
