VEGETABLE PHYSIOLOGY. 



and thus we can readily account for the odours 

 given out by decaying animal and vegetable 

 substances, as they evidently proceed from the 

 volatile parts being liberated by decomposition. 

 The fragrance of flowers, however, escapes while 

 the plants are in a living state, and that most 

 abundantly when they are in vigorous and healthy 

 condition. Besides the flowers, other parts of 

 living plants frequently exhale fragrant odours 

 such as the leaves of the myrtle and geranium, and 

 the wood and bark of pines. All these odours pro- 

 ceed from oily or resinous matters contained in the 

 receptacles of secretion ; but the laws which regu- 

 late their liberation, and define their physiological 

 uses, are as yet imperfectly known. The odours of 

 plants are of three kinds : permanent, fugitive, and 

 intermittent. Permanent odours are those given 

 out slowly by the plant, not only whilst it is living, 

 but also after the fragrant part has been separated 

 from the living plant. Of this kind are the odours 

 of fragrant wood, of the dried petals of roses, and 

 some other flowers. Intermittent odours are the 

 most difficult to be accounted for by the vegetable 

 physiologist. It is well known that the night- 

 smelling stock and several other plants, which are 

 entirely devoid of scent during the day, are 

 delightfully fragrant during the night. One of 

 the orchideous plants produces its powerful aro- 

 matic scent only when exposed to the direct rays 

 of the sun ; and the flower of the night-blowing 

 cereus is fragrant only at intervals during the time 

 of its expansion. 



Tastes. The tastes produced by vegetable sub- 

 stances are generally recognised as sweet, acid, 

 bitter, astringent, austere, or acrid. As a general 

 law, it may be stated that the drier and warmer 

 the situation, the more exposed to light, and the 

 slower the growth of any vegetable, the more 

 intense is its peculiar flavour. 



Luminosity, Heat, Electricity. The luminosity 

 of plants that is, the evolution of light either from 

 living or dead vegetable structure is a rare and 

 curious phenomenon. Flowers of an orange colour, 

 as the marigold and nasturtium, have been occa- 

 sionally observed to present a luminous appearance 

 on still warm evenings ; this light being either in 

 the form of slight electric-like sparks, or steadier, 

 like the phosphorescence of the glow-worm. Certain 

 fungi, which grow in warm and moist situations, 

 produce a similar phosphorescence ; and decaying 

 vegetables, like dead animal matter, have been 

 observed to emit the same kind of luminosity. 

 This phenomenon seems connected with the ab- 

 sorption of oxygen ; and the parts emitting it are 

 said to be most luminous when immersed in pure 

 oxygen, and cease to emit it when excluded from 

 that element. 



The evolution of heat by living plants is a more 

 common phenomenon. We are aware that warm- 

 blooded animals have the power of keeping up a 

 certain temperature within them, which varies at 

 certain stages of their growth, and perhaps period- 

 ically. This result is obtained by respiration the 

 oxygen of the atmosphere uniting with the carbon 

 of their blood, and producing a kind of com- 

 bustion. A similar, though less understood phe- 

 nomenon, seems to take place in the respiration of 

 plants. In germination, heat is sensibly evolved ; 

 a piece of ice placed on a growing leaf-bud will 

 dissolve, when it would remain unchanged in the 

 open air ; and experiment has proved that the 



surface of plants is three or four degrees higher 

 than the surrounding medium. Again, the internal 

 temperature of a lafge trunk is always higher than 

 the surrounding atmosphere, and though young 

 shoots are sometimes frozen through, the general 

 structure both of the wood and bark is such as to 

 conduct heat so slowly, that the internal warmth 

 is seldom reduced beyond what seems necessary 

 to the maintenance of vitality. Generally speaking, 

 it may be asserted that plants possess an internal 

 vital temperature, and that in the so-called process 

 of respiration the giving off of carbonic acid or 

 oxygen, as the case may be a certain degree of 

 heat is evolved ; but precise experimental results 

 are wanting. At the time that the essential organs 

 of flowers are fully developed, a certain amount of 

 heat is given out. This heat is rapidly carried off 

 by the air, and is therefore not easily detected ; but 

 in certain cases, especially in plants belonging to 

 the natural order A races, the elevation of tempera- 

 ture is very marked ; the temperature inside the 

 flowering spathe varying from 10 to 50 F. above 

 the surrounding atmosphere. 



The connection of electricity with vegetable 

 growth has recently excited the attention of physi- 

 ologists ; but little positive information has yet 

 been ascertained. It has been long known that 

 growth takes place with great rapidity during 

 thundery weather; but this may result from the 

 nitrogenised products of the showers which then 

 fall, as well as from the effects of electricity. The 

 progressive states of vegetable growth are the 

 result of chemical changes ; and as these changes 

 are more or less accompanied by electricity, it is 

 supposed that plants evolve electricity as well as 

 heat. The general electric state of plants is said 

 to be negative; and some have attempted to con- 

 nect the luxuriant vegetation of the tropics with 

 the thunder-storms of these regions, on the sup- 

 position that when the atmosphere is positively 

 electrified, the two opposite states will give rise to 

 such commotions. 



SECRETIONS AND EXCRETIONS OF PLANTS. 



Substances of varied properties are secreted 

 by plants, and otherwise formed in their tissues 

 according to their respective natures, and their 

 healthy or diseased condition at the time of secre- 

 tion. Some of these substances are produced by 

 the ascending sap; but the greater number are 

 deposited by the elaborated or proper juice, and 

 consequently are seldom secreted during spring or 

 early summer. The intensity of those derived from 

 the latter source depends in a great measure upon 

 the influence of solar light ; hence they are much 

 stronger, and more abundantly produced, in trop- 

 ical than in temperate climates. From the manner 

 in which many of these are deposited or ejected, 

 they appear to be of little or no utility in the 

 vegetable economy. Some of them may be re- 

 garded as excretions as well as secretions; but 

 whether they are to be considered as essential 

 components of the sap, or evacuations necessary 

 to the healthy condition of the organs, has not yet 

 been determined. Being exceedingly varied in 

 their properties, they are of great utility to man as 

 articles of food, medicine, ornament, and luxury. 



The economical applications of vegetable secre- 

 tions and excretions are so numerous, that it would 

 be impossible, in our limited space, to enter upon 



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