parts of their secretions experience in escaping through the tissue which encloses them, as well as to the degree in which the volatile matter may be fixed. Thus resinous woods, such as Cedar and Cypress, are fragrant for an indefinite period, because the resinous matter in which their odour resides is parted with slowly. Parts whose scent resides in essential oil preserve their scent for a long time, where the essential oil is but slightly volatile, or the wood is thick and hard: thus the Rose-wood of Teneriffe (not the Rose-wood of the English cabinet-makers), produced by Convolvulus scoparius, preserves its odour a very long time; and, in order to elicit it, it is necessary to rub the wood strongly, so as to produce heat enough to volatilise the matter which is locked up in the very compact tissue of which that plant consists. The necessity of producing a little heat, in order to produce an exhalation of the volatile matter, is further exemplified by the fragrance emitted by many woods, otherwise scentless, when exposed to the violent friction of a turner's lathe; Beech is said to acquire, under such circumstances, the smell of roses. But when, on the other hand, the volatile matter is enclosed in wood of a loose texture, neither is heat required to elicit it, nor has the wood, if exposed to the air, the power of retaining it for any considerable time, for the oxygen of the atmosphere will seize upon it rapidly, and quickly leave nothing behind but the inodorous tissue: this happens to Cassia and Cin

namon.

Fugitive smells are those which, belonging to perishable. organs, are either extremely perishable in their very nature, or are placed in tissue of the laxest kind, or are situated on the surface of plants where their volatile parts are continually abstracted by the atmosphere, or finally are secreted in quantities so small that a short exposure to air suffices to dissipate them. All these odours are produced only during the life of a plant; they are dispersed as they are formed, and after death leave no trace of their existence behind them. Like per

manent odours, these are continually given off; and in some plants, as the Orange and the Violet, without any variation in intensity in different states of the atmosphere; but in the majority of cases the power of the smell will vary according

to the elevation of temperature, and the dampness of the air. This fact must be familiar to all who are acquainted with gardens. In the hot dry weather of a summer's noon, flowers either become scentless, or at least lose a large proportion of their usual fragrance; and, in walking through a wilderness of the most sweet-smelling plants, we find little sign of their odour, unless they are bruised or trampled upon. But if a heavy shower should come on, all will be changed in an hour's time; every leaf, every flower, will emit its peculiar odour; the Musk Plant (Mimulus moschatus) will fill the air with its singular scent, and it will be obvious that the addition of moisture to the air has produced a total change in the action of the odoriferous organs of plants.

The same phenomenon is daily repeated in the driest days of autumn. Those only who are accustomed to take their early walks abroad can have any idea of the difference between a richly stored garden early in the morning and at noon. When the sun has dried the air, and has been beating for some time upon vegetation, ill able to bear his action, in consequence of the dryness of the source from which it. draws its means of compensating for evaporation, however beautiful a garden may still remain, it cannot be compared to the same place before the dew has dispersed; when every herb, tree, and flower is pouring forth a stream of the most varied and delicious fragrance; when the air is impregnated with the most delicate balsamic odours; and when all nature seems as if offering up incense in gratitude for the refreshing powers of darkness and of dew. Let any one, for example, visit a thicket of Cistuses at noon, and again the next morning, and the difference will be exceedingly apparent. To what cause this is owing is unknown; possibly the effect of dryness and excessive heat may be to close the stomates, and to contract the tissue of plants, thus rendering it difficult for volatile matter to pass through their cuticle: it may also act by depriving them of the necessary proportion of water required to enable them to perform their functions of secretion and assimilation, and thus arrest for a time the elaboration of the fugitive principles upon which fragrance depends. While, however, dew and showers, with intervals of bright light, are

eminently favourable to the eliciting of vegetable perfumes, a continuance of wet and gloomy weather, without much sunshine, is as greatly unfavourable. This latter circumstance is explicable upon the general law of physiology, that secretions cannot be readily produced without the direct assistance of the sun's light.

With regard to what we call intermittent odours, no explanation seems possible in the present state of our knowledge. A few examples of them will therefore be all that we can give. All dingy-flowered plants, such as botanists call tristes, belong to this class; such as the Pelargonium triste, Hesperis tristis, Gladiolus tristis, which are almost entirely scentless during the day, but become deliciously fragrant at night. Great numbers of Orchideous plants have flowers possessing the same property: the Catasetums have a fine aromatic odour at night, none in the day, except C. purum; Cymbidium sinense is also chiefly fragrant at night; and so with a great many more. Cestrum nocturnum is another plant of the same nature; in the day it has no odour, at night its perfume is extremely powerful. One of the most singular instances of exceptions to all rules appears to be referable to this class: Cacalia septentrionalis exhales an aromatic odour if exposed to the direct rays of the sun; and if any thing is interposed between it and the sun its odour disappears, but is renewed as soon as the interference is removed.

The best observations upon intermittent odours, that I know of, are those of Morren (Observations sur l'Anatomie et la Physiologie de la Fleur du Cereus grandiflorus). He states that in this plant the fragrance is not traceable to any glandular apparatus, or to some reservoir of secretions, but that it is strictly functional, a vital action of the organs of fructification. The fragrance is evidently formed in the organs that part with it; for when an unexpanded flower was cut in two in the morning, being at that time scentless, it became fragrant towards 7 o'clock in the evening. "The odour is undoubtedly formed in each cell of parenchyma by a particular process." It is the property of the Cereus flower to part with its fragrance at intervals only. Morren observed in one case of a cut flower, that it gave off puffs of odour every half hour, from 8 to 12

p. m., when it faded, and the smell became very slight. On another occasion, when the flower was left on the plant, it began to expand at 6 p. m., when the first fragrance was perceptible in the greenhouse. A quarter of an hour afterwards the first puff of odour took place, after a rapid motion of the calyx in rather less than a second quarter of an hour another powerful emanation of fragrance took place: by 35 minutes past 6 the flower was completely open: at a quarter to 7 the odour of the calyx was the strongest, but modified by the petals: after this time the emanations of odour took place at the same. periods as before. Morren considers it probable that these exhalations are periodical, because the emission of carbonic acid by the same organs takes place also in an intermittent manner, and that the emanations of fragrance are a sign of the respiration of the flower.

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BOOK III.

GLOSSOLOGY; OR, OF THE TERMS USED IN BOTANY.

In order to comprehend the language of botanists, it is necessary that the unusual terms or words which are employed in writing upon the subject, and which are either different from words in vulgar use, or which are in Botany employed in a particular sense, should be fully explained.

It is a very common plan to mix up Glossology with Organography, or to confound the definition and explanation of those characteristic terms of the science which are universally applicable, with the description of particular organs: but this plan is attended with many inconveniences, and is far less simple than to treat of the two separately. It was an error into which Linnæus fell, in composing his admirable Philosophia Botanica; and is the more remarkable, if the logical precision with which that work is otherwise composed be considered. Instead of distinguishing those terms which have a general application to all plants or parts of plants, according to circumstances, from such as have a particular application, and relate only to special modifications, he placed under his definition of each organ those terms which he knew to be applicable to it; but, as it was not his practice to repeat terms after they had been once explained, it frequently happened that beginners in the science, finding a given term explained once only, and with reference to a particular organ, fell into the mistake of supposing that that term was applicable only to the organ under which it was explained. To avoid this difficulty, other botanists have col