Igtt] SHREVE—JAMAICAN HYMENOPHYLLACEAE 195 
only as many as 4 or 5 in the finely dissected forms, but as many as 
10 in Trichomanes crispum or 25 in the simple leaves of Tricho- 
manes Hookeri. In Trichomanes rigidum the number of walls 
between the vessels and the marginal cells is 6-8, but an examina- 
tion of the lateral walls shows them to have a peculiar thickening, 
which gives the central part of each one as seen in section the appear- 
ance of having a double convex lens inserted in it (fig. 5). These 
ridges occur in the lateral walls throughout the leaf, so as to pro- 
vide the leaf with a rigid meshwork which is calculated to strengthen 
it in very much the same way that a piece of wire glass is strength- 
ened by the wire netting imbedded in it. Although at every point 
in the lateral walls there are areas of thin wall above and below the 
Fic. 5.—Vertical section of the leaf of Trichomanes rigidum, to show the mechani- 
| cal thickening of the lateral walls; X 764. ; 
thickening, yet it can be readily seen that the thickenings serve 
to hinder the transfer of water from cell to cell, and may be account- 
able for the inability of this species to secure root-absorbed water 
quickly enough to meet even the demands of transpiration in a 
nearly saturated atmosphere. Although Trichomanes capillaceum 
'S quite as hygrophilous a form as T. rigidum, it has to its advantage 
aS Tespects water movement in the leaf the fact that its marginal 
cells are never more than 4 or 5 cells removed from the veins, and 
also that in its slightly thickened walls there are intercellular 
Passages, capable of functioning as avenues for the ready transfer 
of water. ; 
TRANsPrration.—For the measurement of transpiration in the 
Hymenophyllaceae the only method available is that of the 
Potometer, which admits of having the leaf surface wet or dry. 
Attempts to use entire plants on the potometer were not satis- 
factory, and the slenderness of the petioles of most forms, together 
