304 BOTANICAL GAZETTE [APRIL 
drive water through the bundle sheath into the intercellular spaces, 
and through the substitute hydathode into the free air ; but the pres- 
sure thus exerted is much less in proportion to the osmotic pressure 
to which the protoplasm is accustomed in the bundle sheath. 
might be anticipated then that through the substitute hydathode, 
where the excess pressure is relatively great, the water would escape. — 
After what has been said, I think it will be agreed that, given the 
structure of the substitute hydathodes as it is, there is nothing very 
surprising or remarkable in their behavior. It remains, then, to show 
that the structure is essentially similar to such as the same condition 
produces in many plants. The condition is excess of moisture. The 
chief characteristic of the structure is great hypertrophy of a group of 
cells. 
- Two years before Haberlandt first published his account, a paper 
appeared by Atkinson’ on oedema of the tomato. When tomato beds 
in the forcing house were warmed so that root pressure furnished ars! 
faster than it transpired, or when water was forced into branches by 
connection with a faucet, these oedemata were developed. In my 
laboratory, Mr. T. C. Johnson caused their formation readily by fore 
ing water into cut branches under a moderate head of mercury. es -. 
observations agreed entirely with Atkinson’s, whom I ciel OE 2 
_ “The epidermal cells were very much enlarged, while the chlorophy 8 
bearing cells just beneath, as well as some of the more deeply s¢ 
@ cells, were greatly elongated in a radial direction, and strongly ie . 
at their outer extremity where this extended beyond the lateral e. a 
_ Sure from adjacent tissue. In many cases the epidermal cells qui oe 
separate and slough off. The cells of the affected areas possess x" — 
_ ingly delicate walls, so that with little disturbance they panne , : 
There was little protoplasm in proportion to the size of the ee f 
corresponding amount of cell sap.” ‘< The increase in the nul ope 3 
cells in such cases,” 7. ¢., when there is an increase, “ occurs epee 
the formation of transverse walls in the elongated cells.” Later 4 o 
son speaks of “stretching of groups of cells to eight to tem a 
normal size.” The oedemata appear on the veins of the lower’ ae 
of the leaf blade (rarely on the upper surface), on the ae “drops 
and stem. When water is forced into branches it paedee” is a 
along the edges of the leaves, or elsewhere, if any injury *°” 
__ SATKINSON, G. F., Oedema of the tomato. Rep. Agr. Exp: Stat, ™ 
1893, pp. 101-128. oo. 
