STRUCTURE.] SPINES ANOMALOUS LEAVES. 303 



the woody tissue of the veins, or by a contraction of the 

 parenchym of the leaves : in the former case they project 

 beyond the surface or margin of the leaf, as in many Solana 

 and the Holly (Ilex aquifolium) : in the latter they are the 

 veins themselves become hardened, as in the spiny petiole of 

 many Leguminous plants. So strong is the tendency in 

 some plants to assume a spiny state, that in a species of 

 Prosopis from Chili, of which I have a living specimen now 

 before me, half the leaflets of its bipinnate leaves have the 

 upper hah converted into spines. 



The production of spines by hardening veins in the absence 

 of the usual web of parenchym, is a well known occurrence in 

 the Barberry and Gooseberry, whose so-called spines are 

 really nothing but spiny webless leaves. This is still more 

 remarkable in the Puya heterophylla, which bears two kind 

 of leaves having scarcely any resemblance to each other. 

 Those at the base of the plant arise from tough, concave, 

 broad, horny petioles, which overlie each other, forming a 

 kind of bulb, and are extended into narrow, hard, serrated, 

 spiny, brown processes about two inches long. The leaves, 

 on the other hand, which are last formed, are thin, lanceolate, 

 bright green, and more than eighteen inches long when full 

 grown, and bear no resemblance to the first. The flowers 

 are arranged in a close, oblong spike, composed of imbricated 

 woolly cartilaginous pale green bracts, occupying the centre 

 of a bulb of spiny leaves in the place of the thin leaves before 

 mentioned. 



Unusual forms of leaves are not, however, caused exclu- 

 sively by the circumstances above mentioned. Without any 

 peculiar enlargement of the petiole, or the formation of 

 pitchers, or the excessive development of spiny processes, leaves 

 of a very unusual nature sometimes occur ; an example of 

 which (the more interesting because of its frequency) is 

 found among Conifers, which are thus described by Link in 

 that author's admirable Icones Selectee. Anatomic Botanicce, 

 Part ii. tab. 5, (1840). The leaves of Conifers are principally 

 distinguished by having only one simple (occasionally, per- 

 haps double) fibro-vascular bundle, which runs longitudinally 



