From the foregoing results it is evident that boron is very widespread in the vegetable kingdom, entering into the composition of many plants in all the great classes. A general impression obtains that its distribution is universal, and that it will ultimately prove to enter into the composition of practically every plant, as the scope of the analyses is widened and as methods of detection are improved. On the other hand, Agulhon is inclined to think that boron may be a “particular element,” characteristic of certain groups of individuals or of life under certain conditions. The series of individuals differ among themselves as to their particular needs of nutriment (in the widest sense) and doubtless each group has special need of particular elements, a need that is possibly correlated with morphological and chemical differences. It may well be that boron is one of these elements, associated with certain vital functions in a way as yet unexplained, though it may possibly be found to play some part in the formation of vascular tissues, since it is most abundant in bark and lignified parts. II. EFFECT OF BORON ON THE GROWTH OF HIGHER PLANTS. 1. _Toxic effect._ (_a_) _Toxic action of boron compounds in water cultures._ Excessive quantities of boric acid are decidedly poisonous to plants, the action being well marked in water cultures. Knop (1884) found that free boric acid was poisonous in neutral food solutions when present at the rate of ·5 gram per litre, but he was not able to detect boron in the ash of the roots of the experimental plants. Archangeli (1885) placed seedlings of maize, white lupins, _Vicia sativa_ and _Triticum vulgare_ in solutions of boric acid varying in concentration from 1–·05%, with controls in spring water. In the latter case the development was normal, with 1% boric acid the plants were killed, while it was found that the weaker the solution (within the indicated limits) the stronger the root and shoot growth.