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 (V7 Text 
Fig.), shows wide open elements, many of which extend as 
wings, or as wedges of growth, intothe 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 large 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, Senecio precox 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 wet sea- 
son, do not show the typical xerophytic structure, as one would 
naturally expect from their general leathery texture. 
That Sezecto precox 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- 
groundsel. 
