Nicotiana tabacum.—­This plant is covered with innumerable hairs of unequal lengths, which catch many minute insects.  The pedicels of the hairs are divided by transverse partitions, and the secreting glands are formed of many cells, containing greenish matter with little globules of some substance.  Leaves were left in an infusion of raw meat and in water for 26 hrs., but presented no difference.  Some of these same leaves were then left for above 2 hrs. in a solution of carbonate of ammonia, but no effect was produced.  I regret that other experiments were not tried with more care, as M. Schloesing [page 353] has shown* that tobacco plants supplied with the vapour of carbonate of ammonia yield on analysis a greater amount of nitrogen than other plants not thus treated; and, from what we have seen, it is probable that some of the vapour may be absorbed by the glandular hairs.]

Summary of the Observations on Glandular Hairs.—­From the foregoing observations, few as they are, we see that the glands of two species of Saxifraga, of a Primula and Pelargonium, have the power of rapid absorption; whereas the glands of an Erica, Mirabilis, and Nicotiana, either have no such power, or the contents of the cells are not affected by the fluids employed, namely a solution of carbonate of ammonia and an infusion of raw meat.  As the glands of the Mirabilis contain protoplasm, which did not become aggregated from exposure to the fluids just named, though the contents of the cells in the blade of the leaf were greatly affected by carbonate of ammonia, we may infer that they cannot absorb.  We may further infer that the innumerable insects caught by this plant are of no more service to it than are those which adhere to the deciduous and sticky scales of the leaf-buds of the horse-chestnut.

The most interesting case for us is that of the two species of Saxifraga, as this genus is distantly allied to Drosera.  Their glands absorb matter from an infusion of raw meat, from solutions of the nitrate and carbonate of ammonia, and apparently from decayed insects.  This was shown by the changed dull purple colour of the protoplasm within the cells of the glands, by its state of aggregation, and apparently by its more rapid spontaneous movements.

* ‘Comptes rendus,’ June 15, 1874.  A good abstract of this paper is given in the ‘Gardener’s Chronicle,’ July 11, 1874. [page 354]

The aggregating process spreads from the glands down the pedicels of the hairs; and we may assume that any matter which is absorbed ultimately reaches the tissues of the plant.  On the other hand, the process travels up the hairs whenever a surface is cut and exposed to a solution of the carbonate of ammonia.

The glands on the flower-stalks and leaves of Primula sinensis quickly absorb a solution of the carbonate of ammonia, and the protoplasm which they contain becomes aggregated.  The process was seen in some cases to travel from the glands into the upper cells of the pedicels.  Exposure for 10 m. to the vapour of this salt likewise induced aggregation.  When leaves were left from 6 hrs. to 7 hrs. in a strong solution, or were long exposed to the vapour, the little masses of protoplasm became disintegrated, brown, and granular, and were apparently killed.  An infusion of raw meat produced no effect on the glands.