88 Anatomy and Physiology. 



pose as much green surface as possible ; this green matter, or 

 Chlorophylle, as it is called, consists of minute grains lying 

 loose in the cell of the framework of the leaf, and is only formed 

 when exposed to light ; by it alone the sap is purified. Yet in 

 some cases the stem performs the office described. The dodder 

 has no leaves ; many of the cactuses are perpetually destitute 

 of foliage, as well as some euphorbias ; yet in all these cases, 

 the true leaf is still present but unexpanded, the green color of 

 the stem betraying its active agent. Nuttall says, " in the Xylo- 

 phyllum and Ruscus (the butchers-broom), the ambiguity of the 

 stem and leaf attains its utmost ; for the apparent green leaves 

 perform at once the functions both of branches and foliage, the 

 extreme branchlets appearing perfect leaves, but possessing the 

 singular faculty of producing flowers, either on their surface, as in 

 Ruscus, or in the depressions of their margin, a circumstance 

 exhibited in the curious Xylophyllum. In Cypressus Dislicha, the 

 smallest branchlets possess a common function with the leaves, 

 being equally green when growing, and both alike deciduous to- 

 gether. In all these examples it is clear, that the foliage is only 

 dispensed with, when the succulent or herbaceous stem becomes 

 capable of performing the foliar functions ; because the aerating 

 of the sap, and the elaborating it for the maintenance of the other 

 parts of the plant, are the indispensable foliar functions. 



A leaf, then, as will be seen from what we have observed, is 

 nothing more than an expansion of the stem, made up like it of 

 two parts, cellular and woody ; the first, of course, consisting of 

 the green pulp, the second, the framework in which it is laid. 

 The woody matter in the leaf serves the same purpose it did in 

 the stem, giving support and affording tubes for the conveyance 

 and distribution of the sap. When the leaf is expanded hori- 

 zontally, its most general position, it presents two different kinds 

 of surfaces, the one to the sky and the other to the earth. This 

 difference is owing merely to an arrangement of structure for the 

 regulation of its perspiration, for the leaves of aquatic plants 

 undergo changes to meet their situation ; and in the acacias 

 that have true vertical leaves, both sides are precisely alike. 

 These last constitute more than half of the forests of New Hoi 

 land, and are the cause of that peculiar distribution of light and 

 shade for which they are noted, and which was undiscovered til' 

 the time of Robert Brown. 



