the Broad Bean and certain other Plants . 635 
Poisoning effects were not much seen, being usually confined to the 
lower leaves. 
H z BO 3 1 .’500,000. 1:2,500,000 and 1:12,500 000. 
Good shoot development was obtained in all these concentrations of 
boric acid, though the rate of growth was rather slower than where a slightly 
larger quantity had been given. 
H.JIO 3 1:100,000,000. 
As was seen in the case of the root development, such a small quantity 
of boric acid failed to create any apparent difference between these plants 
and the controls : in fact their growth was so similar that this concentration 
was omitted in the last of these three experiments. 
Control: no Boric Acid supplied. 
For the first three weeks growth was apparently normal, and though 
the plants were far behind those treated with boric acid, yet they seemed 
to be healthy. But when the rest of the series were beginning to flower 
(usually after four or more weeks) the control plants presented a very 
abnormal appearance. They were small, stunted, and dark green in colour ; 
the flower buds in many cases withered and fell off, while only a few plants 
bloomed. The apex of the shoot also withered, and the stem at this point 
became blackened (PI. XIII, Fig. 3). This blackening started at the apex 
and travelled slowly down the stem, visible to the eye as black streaks. 
At the same time the leaves were affected. Their texture was leathery 
compared with the leaves of the boron-treated plants, and in many cases had 
the appearance of being covered with small yellow dots. They also showed 
a strong tendency to fall off, the base of the petiole being cut off quite 
clean. Mason ( 22 ) has described a somewhat comparable phenomenon in 
Sea Island cotton, attributing the boll-shedding to a retardation in the rate 
of production of assimilates. Macroscopic examination of the petiole showed 
that the tissue was blackened in a similar way to the stem. A micro¬ 
scopical investigation of the anatomical changes is now being made and will 
be described in a later paper. 
All the control plants did not show this withering at the same time, 
and at the conclusion of one experiment several untreated plants remained 
apparently healthy, though this was unusual. Two such exceptional 
plants were allowed to continue growth for several weeks ; three plants of 
similar age, but which had been continuously supplied with boric acid, were 
also kept on under the same conditions. At the end of a few weeks the 
untreated plants showed the typical ‘ dying off’, while those receiving boric 
acid showed not the slightest indication of it, even after four months’ growth, 
though by this time they had begun to die normally, the lower leaves 
