If individual seeds are the subject of inquiry, then we are assured that the largest seeds of which we have hitherto any experience are the beans of a Mora tree (or as it is now called Dimorphandra oleifera) from Panama. These seeds are as much as six inches long, five inches broad, and four inches thick. If edible, such beans would not be requisite in any great numbers for an ordinary meal.
Justification might almost be found for an allusion to such large starchy roots as the elephant's foot, and yams of various species, in which great bulk is combined with farinaceous qualities, which render them available, after the manner of gigantic potatoes, as articles of animal food.
Those truly elegant plants the Ferns, as popular as any of the members of the vegetable kingdom, have also their giants in the tree ferns of tropical climates. The "silver king" (Cyathea dealbata) has leaves, or fronds, from five to seven feet in length; and Dieffenbach found it growing in New Zealand with trunks upwards of forty-two feet in height. Another, which might be called the "monarch" (Dicksonia antarctica), has fronds from six to twelve feet in length, or more. One plant, cultivated in this country, and hence probably inferior in size to those growing in its native home, is said to have produced fronds eleven feet in length and three feet two inches in width. This plant had altogether fifty fronds, which covered an area of eighteen and a half feet! In Tasmania this fern forms the great feature in the fern valley. Humboldt considers it singular that no mention is made of arborescent ferns in the classic authors of antiquity, the first distinct reference being by Oviedo, in the early part of the sixteenth century. However graceful and elegant some of the palms may be in their foliage and the grandeur of their crested forms, these cannot be compared for beauty with the deeply-cut and infinitely diversified and subdivided fronds of the larger ferns. All that the palms may claim for excess in height, or bulk of trunk, over the tree ferns, is amply compensated in the latter by the beauty and grace of their crown of feathery fronds.
Seaweeds are the most gigantic of cryptogamic plants, and of these the most noteworthy is the large Macrocystis of the antarctic seas (Macrocystis pyrifera). D'Urville says that it grows in eight, ten, and even fifteen brasses of water, from which depth it ascends obliquely, and floats along the surface nearly as far; this gives a length of 200 feet. Dr. Hooker (now Sir Joseph) says: "In the Falkland Islands, Cape Horn, and Kerguelen's Land; where all the harbours are so belted with its masses that a boat can hardly be forced through, it generally rises from eight to twelve fathom water, and the fronds extend upwards of one hundred feet upon the surface. We seldom, however, had opportunities of measuring the largest specimens, though washed up entire on the shore; for on the outer coasts of the Falkland Islands, where the beach is lined for miles with entangled cables of Macrocystis, much thicker than the human body, and twined of innumerable strands of stems coiled together by the rolling action of the surf, no one succeeded in unravelling from the mass any one piece upwards of seventy or eighty feet long; as well might we attempt to ascertain the length of hemp fibre by unlaying a cable. In Kerguelen's Land the length of some pieces which grew in the middle of Christmas Harbour was estimated at more than three hundred feet." He afterwards alludes to what he considered the largest specimens seen, in what is believed to be forty fathoms water, and streaming along the surface, to a probable total length of about 700 feet. The report that this seaweed sometimes attains a length of fifteen hundred feet is probably exaggerated, although it may be true that "it grows up from a depth of forty-five fathoms to the surface, at a very oblique angle, and even when of no great breadth, make excellent natural floating breakwaters."
None of the remaining cryptogamia attain to any extraordinary size. Neither floating mosses nor dendritic forms exceed two or three feet; and lichens only extend to about the same dimensions in the most exaggerated examples. Fungi have not yet produced a Titanic species, for the largest agaric yet known is inferior in expanse to a lady's parasol; and the great puff ball (Lycoperdon giganteum) has not yet attained the dimensions of a somnolent sheep. Amongst the lower cryptogamia we have many examples of the infinitely little, but not of the infinitely great.
Whether we study plant life in its largest or its most minute manifestations, in its simplest or most eccentric forms, through its normal development or exhibiting strange phenomena, we are induced to join with Horatio Smith in his exquisite hymn—
'Neath cloistered boughs, each floral bell that swingest And rolls its perfume on the passing air, Makes Sabbath in the fields, and ever ringest A call to prayer.
Not to the domes, where crumbling arch and column Attest the feebleness of mortal hand, But to the fane, most catholic and solemn, Which God hath planned.
To that cathedral, boundless as our wonder, Whose quenchless lamps the sun and moon supply; Its choir the wind and waves, its organ thunder, Its dome the sky.
There, as in solitude and shade I wander, Through the green aisles, or stretched upon the sod, Awed by the silence, reverently ponder The ways of God.
CHAPTER XVII
TEMPERATURE.
WITHOUT concerning ourselves greatly as to the general temperature of plants, we may premise that the accepted opinion is in favour of the conclusion that it is more equable than that of the surrounding air; that at night, or in winter, it is above, and in mid-day, or in summer, it is below the atmospheric temperature. Most of those who have made experiments have come to the conclusion that trees with thick trunks have a temperature lower than that of the air during great heat, and higher during extreme cold. Dr. Hooker made some observations in India, and was of opinion that the temperature of the fluids in a plant coincided with that of the soil at the spot whence the largest absorption was derived. That a shaddock fruit maintained the same temperature at mid-day with the atmosphere at 110°, as at midnight with the thermometer at 68°. He remarked that, "when the surface sand in the Soane Valley was heated to 110° the fresh juice of Calotropis plant was only 72°. This latter temperature he found at fifteen inches depth in the soil where the plant grew. The power which the plant has in maintaining a low temperature of 72°, though the main portion, which is subterranean, is surrounded by a soil heated between 90° and 100°, is remarkable, and is no doubt proximately due to the rapidity of evaporation from the foliage, and consequent activity of the circulation. Its exposed leaves maintained a temperature of 80°, nearly 25° lower than the similarly exposed sand and alluvium." The inference is, that the liquids taken up by the roots, being at the degree of heat which the soil possesses, at that depth tends to warm the tree in the cold season, and to cool it, in comparison with the air, in the warm season.
Apart from this question of general temperature we are concerned chiefly with the great increase of heat evolved by plants under certain conditions, especially at germination, and during flowering. That is, the phenomena of increased temperature under special circumstances. In animals the heat of the body is maintained by a process analogous to combustion. Oxygen combines with carbon and forms carbonic acid, which latter is thrown off, the change or oxidation being accompanied by the evolution of heat. As it is in the combustion of carbon so is it in the conversion of carbon in the animal body, and so also in plants, under special conditions, when oxidation is greatly increased heat is evolved, chemical changes take place, and the burning log, the breathing animal, and germinating plant all exhibit the same phenomenon of carbon in combustion.
A familiar example of the evolution of heat during germination is furnished in the process of "malting" the grain of barley. Growth is stimulated by moisture, and a large number of seeds being collected together it is easy to experience the increase of temperature caused during the process. The chemical change which the seeds undergo, the absorption of oxygen, the state of slow combustion, the amount of heat evolved, are all easily demonstrated. Thus we ascertain that the change is a chemical one, the starch of the seed by acquiring oxygen becomes soluble and saccharine, this kind of decomposition being accompanied by increase of temperature. The process is essential to the growth of the plant. The starch was insoluble, and therefore incapable of nourishing the young embryo. By acquiring oxygen it becomes soluble and growth proceeds, until checked artificially by drying, and the starchy "barley" is converted into the sugary "malt." That which is here effected artificially is simply the ordinary course of nature.
From this process we learn that there is a chemical change, accompanied by evolution of heat, to a greater or less extent, in all seeds during germination. So, also, at a subsequent period, namely, that of flowering, certain chemical changes take place, which are equivalent to decomposition, in which oxidation takes place, and heat is evolved during the process.
We are chiefly concerned here in the phenomenon of the evolution of heat at the time of flowering, for although, as in the case of germination, it undoubtedly takes place, more or less, in all plants, it is only under favourable conditions that the temperature is raised to an appreciable extent. The most suitable condition for observing the heat evolved during germination is when a large number of seeds are collected together; so, also, the most favourable condition for the determination of the amount of heat evolved at the period of flowering is when a large number of flowers are associated together. This will account for the high temperature determined in certain plants to be presently alluded to, the results being proportioned to the number of associated flowers.
The evolution of heat at the time of flowering has been observed most frequently, and with the greatest satisfaction, in plants of the arum family, in which a large number of flowers are collected together at the base of the spadix, and these are surrounded by and enclosed within an envelope, or spathe, which prevents the rapid dissemination of the heat engendered. This structure is sufficiently represented in our common indigenous Arum maculatum, called "Lords and Ladies," for illustration (fig. 83). This phenomenon was first observed by Lamarck, in 1777, but without any precise determination of the heat experienced. In 1800, Sennebrier measured with a thermometer the heat evolved in the common arum, and found it 86° cent. In the early part of the same century Hubert states that a thermometer placed in the centre of five spadices of Arum cordifolium, in the Isle of France, stood at 131° Fahr., and in twelve at 142 1/2° Fahr., while the temperature of the air was only 74-75°. This showed an elevation of 56° and 68°! Schultz observed the flowers of Caladium pinnatifolium, at Berlin, in 1828, and M. Treviranus published the results of investigations on several species of Arum, in 1829. Goeppert, in 1832, found the temperature of the spadix of Arum dracunculus rose to 31° Fahr., above the temperature of the surrounding air. In 1834 M. Brongniart observed the elevation of temperature in Colocasia odora as 19.8° Fahr., above that of the conservatory in which it was growing. Van Beek and Bergsma examined the same species in 1828, and found an elevation of 50° Fahr., above the surrounding air, by means of a thermo-electric apparatus.