188 SCIENCE OF GARDENING. Part II. 



cereus. But it seems somewhat doubtful whether or not light is the sole agent in the present" case ; for 

 it has been observed that equatorial flowers open always at the same hour, and that tropical flowers 

 change their hour of opening according to the length of the day. It has been observed also, that the 

 flowers of plants that are removed from a warmer to a colder climate expand at a later hour in the latter. 

 A flower that opens at six o'clock in the morning at Senegal, will not open in France or England 

 till eight or nine, nor in Sweden till ten. A flower that opens at ten o'clock at Senegal, will not 

 open in France or England till noon or later, and in Sweden it will no* open at all. And a flower 

 that does not open till noon or later at Senegal, will not open at all in France or England. This seems as 

 if heat or its absence were also an agent in the opening and shutting of flowers ; though the opening of 

 such as blow only in the night cannot be attributed either to light or heat. But the opening or shutting 

 of some flowers depends not so much on the action of the stimulus of light as on the existing state of 

 the atmosphere, and hence their opening or shutting betokens change. If the Siberian sow-thistle shuts 

 at night, the ensuing day will be fine ; and if it opens, it will be cloudy and rainy. If the African mari- 

 gold continues shut after seven o'clock in the morning, rain is near at hand. And if the convolvulus ar- 

 vensis, calendula fluvialis, or anagallis arvensis, are even already open they will shut upon the approach 

 of rain, the last of which, from its peculiar susceptibility, has obtained the name of the poor man's 

 weatherglass. But some flowers not only expand during the light of day ; they incline also towards the 

 sun, and follow his course, looking towards the east in the morning, towards the south at noon, and 

 towards the west in the evening ; and again returning in the night to their former position in the morn- 

 ing. Such flowers are designated by the appellation of Heliotropes, on account of their following the 

 course of the sun ; and the movement they thus exhibit is denominated their nutation. This phenomenon 

 had been observed by the ancients long before they had made any considerable progress in botany, and 

 had even been interwoven into their mythology, having originated, according to the records Cf fabulous 

 history, in one of the metamorphoses of early times. Clytie, inconsolable for the loss of the affections of 

 Sol, by whom she had been formerly beloved, and of whom she was still enamoured, is represented as 

 brooding over her griefs in silence and solitude ; where, refusing all sustenance, and seated upon the 

 cold ground, with her eyes invariably fixed on the sun during the day, and watching for his return dur- 

 ing the night, she is at length transformed into a flower, retaining, as much as a flower can retain it, 

 the same unaltered attachment to the sun. This is the flower which is denominated heliotropium by the 

 ancients, and described by Ovid as Flos qui ad solem vertitur. But it is to be observed, that the flower 

 alluded to by Ovid cannot be the heliotropium of the moderns, because Ovid describes it as resembling 

 the violet : much less can it be the sun-flower of the moderns, which is a native of America, and could 

 not consequently have been known to Ovid ; so that the true heliotropium of the ancients is perhaps not 

 yet ascertained. Bonnet has further remarked that the ripe ears of corn, which bend down with weight 

 of grain, scarcely ever incline to the north, but always less or more to the south ; of the accuracy of 

 which remark any one may easily satisfy himself by looking at a field of wheat ready for the sickle ; he 

 will find the whole mass of ears nodding, as if with one consent to the south. The cause of the pheno- 

 menon has been supposed to be a contraction of the fibres of the stem or flower-stalk on the side exposed 

 to the sun ; and this contraction has been thought by De la Hire and Dr. Hales to be occasioned by an 

 excess of transpiration on the sunny side; which is probably the fact, though there seems upon this 

 principle to be some difficulty in accounting for its returning at night; because if you say that the con- 

 tracted side expands and relaxes by moisture, what is it that contracts the side that was relaxed in the 

 day? The moisture, of which it is no doubt still full, would counteract the contraction of its fibres, and 

 prevent it from resuming its former position in the morning. 



847. Heat as well as light acts also as a powerful stimulus to the exertion of the 

 vital principle. This has been already shown in treating of the process of germination ; 

 but the same thing is observable with regard to the developement and maturation of the 

 leaves, flower, and fruit ; for although all plants produce their leaves, flower, and fruit, 

 annually, yet they do not all produce them at the same period or season. This forms 

 the foundation of what Linnaeus has called the Calendarium Florce, including a view of 

 the several periods of the frondescence and efflorescence of plants, together with that of 

 the maturation of the fruit. 



848. Frondescence. It must be plain to every observer, that all plants do not protrude their leaves at the 

 same season, and that even of such as do protrude them in the same season, some are earlier and some 

 later. The honeysuckle protrudes them in the month of January ; the gooseberry, currant, and elder, 

 in the end of February, or beginning of March ; the willow, elm, and lime-tree, "in April ; and the oak 

 and ash, which are always the latest among trees, in the beginning or towards the middle of May. 

 Many annuals do not come up till after the summer solstice ; and many mosses not till after the com- 

 mencement of winter. This gradual and successive unfolding of the leaves of different plants seems to 

 arise from the peculiar susceptibility of the species to the action of heat, as requiring a greater or less 

 degree of it to give the proper stimulus to the vital principle. But a great many circumstances will al- 

 ways concur to render the time of the unfolding of the leaves somewhat irregular ; because the mildness 

 of the season is by no means uniform at the same period of advancement ; and because the leafing of the 

 plant depends upon the peculiar degree of temperature, and not upon the return of a particular day of 

 the year. Hence it has been thought, that no rule could be so good for directing the husbandman in the 

 sowing of his several sorts of grain as the leafing of such species of trees as might be found by observation 

 to correspond best to each sort of grain respectively, in the degree of temperature required. Linnaaus 

 (Stillingfleet informs us) instituted some observations on the subject about the year 1750, with a view 

 chiefly to ascertain the time proper for the sowing of barley in Sweden ; he regarded the leafing of the birch- 

 tree as being the best indication for that grain, and recommended the institution of similar observations 

 with regard to other sorts of grain, upon the ground of its great importance to the husbandman, who 

 may be said to attend to it in a manner instinctively ; but as all the trees of the same species do not come 

 into leaf precisely at the same time, and as the weather may alter even after the most promising indi- 

 cations, no guide natural or artificial can be absolutely depended on with a view to future results. 



849. Efflorescence. The flowering of the plant, like the leafing, seems to depend upon the degree of tem- 

 perature induced by the returning spring, as the flowers are also protruded pretty regularly at' the same 

 successive periods of the season. The mezereon and snowdrop protrude their flowers in February ; the 

 primrose in the month of March ; the cowslip in April ; the great mass of plants in May and June ; many 

 in July, August, and September ; some not till the month of October, as the meadow saffron ; and some 

 not till the approach or middle of winter, as the laurustinus and arbutus. Such at least is the period of 

 their flowering in this country ; but in warmer climates they are earlier, and in colder climates they are 

 later. Between the tropics, where the degree of heat is always high, it often happens that plants will 

 flower more than once in the year ; because they do not there require to wait till the temperature is 

 raised to a certain height, but merely till the developement of their parts can be effected in the regular 

 operation of nature, under a temperature already sufficient. For the greater part, however, they flower 

 during our summer, though plants in opposite hemispheres flower in opposite seasons. But in all climates 

 the time of flowering depends also much on the altitude of the place as well as on other causes affecting 

 the degree of heat. Hence plants occupying the polar regions, and plants occupying the tops of the high 

 mountains of southern latitudes are in flower at the same season ; and hence the same flowers are later 



