experimenters, are thought to show, that oxygenous gas, or vital air, is excreted from the leaves of plants. This doctrine, however, has, of late, been ingeniously controverted by Professor Woodhouse*.

Ir has also been ascertained, that excretions, of different kinds, take place, during the night, from the roots of plants. It is, indeed, highly probable, from what has already been said†, that some of the radicles, as they are generally denominated, of the bulbous and other roots, are exhaling rather than absorbing vessels. The observations of Professor Brugmans, of Leyden, render it probable, that theliquids which are exhaled from the roots of some plants are very injurious to other plants, which grow near to them. In this way, the Wheat is injured by the Blue Erigeron, or Fleabane (Erigeron acre); Oats by the Corn-Saw-wort, or Way-thistle (Serratula arvensis), and the Carrot (Daucus Carota) by the Elecampane (Inula Helenium). Mr. Brugmans carefully examined the roots of some plants, which had been injured by Darnel. These roots assumed the appearance of having been preyed upon by an insect. With the view to determine, whether the appearance in question was owing to the Darnel, the Professor placed this plant and another in a glass vessel. Upon examining the plant, he found its roots affected, as in the former instance. He, therefore, thought himself authorized to infer, that the injury had been occasioned by the influence of some active fluid, excreted, or emitted from the radicles of the Darnel.

It is much to be wished, that a subject so curious and important as this were pursued, in a train of care

• See Mr. Nicholson's Journal of Natural Philosophy, Chemistry, and the Arts. Vol. ii. For July, 1802.

See Part i. page 12.

fully conducted experiments. The results could not but prove highly interesting in an agricultural point of view.

VII. SECRETORY vessels. The existence of secretory vessels in plants cannot be doubted, although our knowledge of this system is far from being complete. The office of these vessels is to secrete fluids of differ ent kinds, which are useful in the economy of the vege table. They are generally supposed to be accompanied by a glandular apparatus. In the vegetable, however, as well as in the animal, we often find peculiar (and probably secreted) fluids, although the existence of a glandular structure has not been detected. The honied fluid* which is found in the true nectaries, as in those of Canadian Columbine (Aquilegia canadensis), and many other plants, and also in the tubes of many monopetalous corollas, such as that of Jamestown-weed (Datura Stramonium), is, in all probability, secreted by a peculiar action of the vessels of the parts. The anthers are considered as glands, destined to secrete the pollen, or fecundating powder. But these are points still involved in great uncertainty. Much, indeed, remains to be done before the structure, and offices of the various systems of vessels in plants can be completed. A rich and ample field is opened for the labours of some future, happy experimentert.

• See Part 1. pages 145, 146, 147, &c. &c

For further information concerning the vessels of plants, see Part i. pages 46-61. also, pages 80, 81, 82, 140, 141, 142.

THE END OF THE SECOND PART.

ELEMENTS OF BOTANY.

PART THIRD.

Systema itaque est acus illa magnetica, quae ex immenso "Rerum naturalium Oceano in portum nos ducit; est spe"culum nitidissimum, quod uno quasi intuitu cujusque "Individui essentialem characterem, veramque imaginern "nobis ostendit; est denique Index rerum praecipuarum, quae ad Historiam naturalem rite addiscendam requirun

"NON EQUIDEM CONTENDO, SCIENTIAM NATURAE 66 SOLO SYSTEMATE LOCUPLETARI."

SCOPOLI.

THE SEXUAL SYSTEM of Linnæus is founded, as I have already observed, upon the beautiful doctrine of the SEXES OF PLANTS. According to this system, all known plants are distributed into different CLASSES, ORDERS, GENERA, SPECIES, and VARIETIES. The classes are twenty-four in number, and are formed from the NUMBER, the PLACE OF INSERTION, the PROPORTION, the CONNECTION, the DISPOSITION, or the ABSENCE of the stamens, or male organs of generation. The orders, or secondary divisions, are much more numerous, and in the first thirteen classes. are founded exclusively upon the number of the styles, or female organst. In the remaining eleven classes, they are founded upon other circumstances, the principal of which have, indeed, already been mentioned, but will be more advantageously explained, in the particular illustration of those orders.

See Part I. p. 170.

+ See Part I. page 178.