40 Harshberger — Water Storage and Conduction. 



The phloem patches of the stem are more or less confluent, 

 forming a cylindrical zone about the wood. The wood ( TT'Text 

 Fig.), shows wide open elements, many of which extend as 

 wings, or as wedges of growth, into the pith. Pitted and spiral 

 elements are present, the latter internal. The bundles do not all 

 extend so deeply into the pith ; only the primary or first-formed 

 bundles do so, their protoxylem dipping into the pith cylinder. 



A surface examination of the lower and the upper epidermis 

 of the leaf reveals no peculiarities of importance. The absence 

 of stomata on the upper, and their presence on the lower, sur- 

 face is noticeable. They are quite large with crescent-shaped 

 guard cells, each with a larc^e nucleus. There are no tricho- 

 mic structures on the leaf. A cross section reveals the absence 

 of a true palisade tissue, the mesophyll being of rounded cells 

 containing chlorophyll and clearly placed together without 

 large intercellular spaces. The stomata open into narrow 

 chambers, which are in communication by narrow connecting 

 passage-ways. There is, therefore, but little of the so-called 

 loose parenchyma. The stomata are not sunken, nor peculiar, 

 although the guard cells are of a good size. These anatomi- 

 cal peculiarities show us, that as regards the roots and the 

 stem, Scnccio pracox is well protected against the dry season 

 and can lay up a store of water in the pith for use during the 

 period of drouth. The leaves, as they appear in the vv"et sea- 

 son, do not show the typical xerophytic structure, as one would 

 naturally expect from their general leathery texture. 



That Scnccio prcvcox is well adapted to grow under the con- 

 dition of climate presented in the Valley of Mexico needs no 

 further proof than what has already been presented. One is 

 always impressed in studying the vegetal kingdom by the dif- 

 ferent methods adopted by plants in securing the same end. 

 The cacti of Mexico, and other succulents of that region, secure 

 immunity from drouth by consolidation and by reduction of 

 transpiration surface, as does likewise our plant, the tree- 

 crroundsel. 



