12 MEMOIRS NATIONAL ACADEMY OF SCIENCES, VOL. XI. 
LEAVES.1 
Among the leaves of such well-known cultivated species as have been named, ratio between 
length and breadth, basal and apical narrowing, concavity, stiffness, and direction of growth 
are fairly constant. In other species there may be great differences (compare plates 36, 37, 
and 107—Agave barbadensis, plates 45 and 46—A. sobolifera, plates 67 and 68—A. Under- 
woodii, and the plants contrasted on plates 76 and 79—A. portoricensis), though reason has 
scarcely been found for seeing more than individual or racial differences in either case. In 
these respects, too, hopeless contradictions and confusions were exhibited in the foliage of 
very young plants of the Bahamian species when their study was first taken up, and it may be 
that within this group apparently characteristic differences in the width of mature leaves, as 
shown by material now accessible, will prove inconstant. Within limits, smoothness or rough- 
ness of the leaf surface, as well as its coloration and the presence or absence of glaucousness, 
are indicative characters; but though some well-known species are persistently smooth, and 
others as persistently granulated, there are others in which a characteristic slight roughening 
varies into either extreme. 
Very significant and constant, in the cultivated species referred to, are the characters pre- 
sented by the apical spine (pl. B) and the sinuation and marginal arming of the leaves. In these 
features, indeed, seems to lie great, if not the greatest, stability in the leaf characters of the genus. 
Here, too, however, there are differences which must be understood and taken into consideration, 
for in repandness the leaf margins of spontaneous plants are found to differ to a very consid- 
erable extent, and the size and the form of basal widening of the prickles vary according to 
the amount of marginal tissue that has sclerified in their development. Contrasts of this sort 
are afforded by Agave vwipara and A. petiolata (pls. 2 and 8), different individuals of A. sobolifera, 
or the prickles of a single leaf of A. Boldinghiana (pl. 12) or of A. Legrelliana (pl. 59), ete. 
The terminal spines of a species often show differences in stoutness which correspond with 
variations in the general form of the leaf tip; and the partial or complete evolution of the 
spine, sometimes depending on the chance of environment, may lead to marked differences in 
its structure and appearance. In this respect the groups of Agave sometimes differ greatly. 
It is in those forms with a determinate or clearly limited spine that this becomes most charac- 
teristic, for a definite part of the leaf tip is here devoted to spine formation, and its cells become 
hardened and colored early. Among native West Indian species indeterminate spines are the 
rule. In cultivation, species with an indeterminate spine often fail to develop more than a 
very short apical point of hard tissue, and A. sobolifera is often little pungent in gardens, while 
its wild representatives possess a strong though variable spine. No really sharp line may be 
drawn, however, between determinate and indeterminate spines, and in the species producing 
the latter a time usually comes, if they are well developed, when continued intrusion into the 
fleshy leaf tissue ceases, and the lower part, already dried, becomes nearly or quite as hard 
and deep in color as the apical part, or as an entire determinate spine. 
The often very characteristic ventral grooving of the spine is greatly dependent on the 
chances attending the development of an indeterminate spine, for while a prompt basal hardening 
insures its perfection, the failure of such a hardening often results in its being pinched into a 
V-like form, a frequent occurrence in the most typical Antillanae and Bahamanae, though 
sometimes without uniformity of result. It is because of indeterminate development that the 
spines of the Caribaeae become so greatly and unequally thickened at the base; and the marked 
contrasts between slit-grooved and open-grooved spines in some forms of A. Underwoodii (pls. 
69 and 70) and A. portoricensis (pls. 78 and 80) are apparently to be explained in no other way. 
In addition to variations in the straightness of the leaf margin between the prickles, in 
its repandness, or in its abruptly hummocky appearance when such species as Agave Karatto 
(pl. 14) and A. Harrisia (pl. 50) or A. Legrelliana (pl. 59) are compared, the margin itself affords 
characters of considerable constancy and value for the recognition of species. In the subgenus 
Tittaea a convenient basis of classification is found in the connection of the prickles by a horny 

1 Engelmann, Trans. Acad. Sci. St. Louis, vol. 3, 1875, p. 294; Bot. Works, 1887, p. 302. 
