47 
lUtlf p. affine, P. Phijmatodes, P. pahuatum, P. alhldo-sqnamaium, 
P. macro phylhim, P. heraclenm, Dryotitarhyvm and Photinopfrris: 
AVluto incrustations of lime arc regularly found on those hydutliudos on 
some si^ecies of Nephrolcpls, P. albldO'Squamaiiun, and joung fronds of 
J)njOf<Uichijinti and Pliofinopteris and occasionally on various otlior ferns. 
Tliero are otlier liyaline vein-tips, as in Asplnnnfu suhiKfnttdIc, which are 
not active hydathodes, though perhaps potential ones. 
Venation. — Except as it is modified by corridation w 1th other slnielni'al 
peculiarities^ such as the fineness of dissection of iho froud^ the venntion 
in general is decidedly closer in species of arid than in those of humid 
habitats. Thus, among i)lants of arid places, the distance between 
veinlets is : 
Of Nephroilium cuvuUuiuni, 0.35 iniiJiiiicler; "N. aridum, 0.4 inilHjiicler; rtcrls 
opaca, 0.19 niillinu'tcr; P. lofHjifoliH, 0.5 Tnilliiiit>ter; /*. melanocaulon, 0.5 mil- 
limeter; among pinnts of moister plares; of .V. frrox, 0.75 iTiilliincter; N. Foxil, 
0.5 iiiillinu'ter; A~. si/nnaticum, 0.7 nullinu'ttT; .Y. pteroidcs, 0.7 milliiiH'tcr; 
lAndsaya scandcns, 0.7 millimeter; i^t/ityra/nma, 1.5 milliim;tors; Diplazium pal- 
lidum, 1 millimeter; A.^plcnivm caudatin}i, I millimeter; Scolopendrium piruuilum, 
2 millimeters; Stcnocftlrma sithfrifoliafa, 2 millimeters; Pteris pfurirvtitdfita, 
0.7 millimeter; Pteris ear-lsa, 1 millimeter. 
Anastomosis of the veins makes the venation closer in ed'ect, and as 
a general proposition, with many exceptions, ferns with anastomosing 
veins arc more xerophytic in habitat than those witli free veins. As 
illustrations on the largest scale, Goniopteris and CalUplcris are dominnut 
in the savanna-wood where LasLrwa and Diplazium do not occur, and 
Phymatodes and it:^ offshoots have fifteen species in the high forest, but 
Eu-Polypodium is unrepresented. The frequent correlation between large 
size and ampleness of frond and reticulate venation is too obvious to need 
elaboration. 
^ I 
Articulate stipe. — The articulation of the stipe to the rhizome, and of 
the i^inrnp or segments to the stipe, facilitate the reduction or removal of 
the leaf surface whenever it is necessary. It is thus an adaptation to life 
where plants must sometimes endure a more or less prolonged want of 
water. Like other adaptive characters, hut in greater mcasuro than 
many, because it involves a deeper specialization, it lias a taxonomic 
value, as species, genera, and even larger groups have de\eloj)ed in 
constant adaptation to certain conditions. Under the conditions at Sail 
Ramon (and imder tropical conditions in general), then, the charncter- 
istically epiphytic groups have articulate stipes; the characteristi<ally 
terrestrial ones, nonarticulate stipes. DacaUlece and Polypodicoi are 
tvpically cpij)hytic tribes with articidate stijies; Asjdcnietr^ Aspidira' and 
Pteridoa', tvi)icallv terrestrial tribes, withr)ut articulations. 
It is the exceptions to this general rule for the tribes which p\it this 
interpretation of articulations beyond any question. Among tlie A.spid- 
lea', we have at San T»anion a single epiphyte, yephrodium 1712, and 
