136 BOTANICAL GAZETTE [avcust 
extremely small amount of ionic H necessary to kill the roots of 
seedlings. In the case of the Pisum sativum seedlings one part of 
ionic H- to 6,400,000 of water was sufficient to kill the roots. 
The roots of Cucurbita Pepo were killed by 1 part of ionic 1 
to 3,200,000 parts of water, while the roots of Zea Mais were 
the most resistant, requiring one part of ionic H to 1,600,000 
parts of water. 
_ When expressed in the form of per cent. the extremely small 
amount of acid necessary to kill the Peswm sativum seedlings 1s 
even more apparent, and may be expressed as follows: HCl, 
9.00056%; H,SO,, 0.00076%; HNO,, 0.00098%; HBr, 
0.00126%. 
From this it will be seen that the per cent. according t0 
weight gives a different result, showing apparently a difference 
in the toxic power of the acids, which would be obtained if the 
theory of dissociation was overlooked. 
Before leaving the subject of acids and their toxic effect 4 
few points in connection with the relation of the plant to CO, are 
worthy of note. All CO, which reaches the plant, whether it be 
the root or the green aerial parts must be brought into solution 
sary to produce a toxic effect is quite large, and this may PF . 
urged as an objection to the ionic explanation. This cai 
ever be easily explained. The CO, absorbed by a leaf is eit . 
up with some difficulty, the resistance depending UP? a 
structural porosity of the leaf and upon the permeability of the : 
cell walls, so that it takes a very considerable external pressut 
or a large per cent, of CO, in the surrounding air to cause He 
