590 Hill . — The Projection of Hairs on the 
in September, both in the open and under the bell-jar, hairs were noticed on 
the formerly glabrous plants in three days’ time (laminae removed 12 o’clock, 
Sept. 12, 1911, examined Sept. 14, 1911, and a few developing laminae re- 
moved, no hairs seen ; Sept. 15, 1911, scattered hairs especially on younger 
parts. Rain fell on the night of 13th and air cool). Again, in October, the 
experiment was repeated during cold, dull, foggy weather (Oct. 14-16), and 
a plant under a bell-jar deprived of all its laminae developed a fairly dense 
felt of hairs on the younger parts in a space of three days. A similar plant 
in the open developed hairs, but to a less extent. Control plants with the 
laminae undisturbed, grown both in the open and under a bell-jar, showed 
no trace of hairs. It was noticed, however, that under a bell-jar a very few 
hairs were developed occasionally if the plant had been covered for about 
two weeks. The plants which had been stripped of their laminae were 
continued in this condition by removal of all developing leaves for about 
ten days, with the result that quite a dense felt of hairs was developed, 
clearly visible to the naked eye. In the case of the plants under a bell-jar the 
hairs were much longer than those of plants in the open, and hairs measured 
from 0*5-1 mm. in length. If, after this copious development of hairs has 
proceeded for some days, the plant is again allowed to grow normally and 
the laminae are not removed, the production of hairs gradually diminishes, 
and in a short time ceases, with the result that the stems and petioles are 
again as glabrous as when the experiment was started. The hairy condition 
can be again induced in the growing portions by removal of the laminae, 
and the glabrous by their retention, so that by the alternate removal and 
retention of the laminae the hairy or glabrous condition may be produced. 
Should, however, the leaves which are allowed to form after a glabrous 
period be malformed and remain more or less aborted, the stem and petioles 
will still retain their hairy character. 
On a plant which had been kept defoliated for some weeks under 
a bell-jar out of doors, it was noticed that hairs had developed on the mid- 
ribs and under sides of the young leaves which were allowed to form at the 
end of October. 
The hairs in T. peregrinum are unicellular, the outgrowth of a single 
epidermal cell. In a transverse section of the stem of a glabrous plant all 
the epidermal cells appear to be identical in character, and no cells can be 
distinguished as potential hair cells. When, however, hairs are about to be 
developed, certain epidermalcells become filled with dense granular contents, 
and such cells, at a slightly later stage, form small protruding papillae which 
will grow out to form the long unicellular hairs. 
The full-grown hair has thin walls, and contains a copious supply ot 
granular vacuolated protoplasm with a conspicuous nucleus suspended either 
near the base or towards the middle of the hair. The apex of the hair is 
rounded and slightly swollen, which makes it appear somewhat glandular, 
