OF VEGETABLK VITALITY. 



155 



that the ripe ears of corn which bend down with 

 the weight of grain, scarcely ever incline to the 

 north, but always more or less in a southerly 

 direction; 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 immediate cause of 

 the phenomena, has been supposed to be a con- 

 traction of the fibres of the stem or flower stalk, 

 on the side exposed to the sun; and the contrac- 

 tion has been thought by De la Hire, and Dr 

 Hales, to be occasioned by an excess of transpira- 

 tion on the sunny side, which is probably near 

 the truth. 



Heat. A certain medium degree of heat is 

 essential to the first development of the vegetable 

 germ, and to the future life of the plant. 



No plants will vegetate at a temperature so 

 low as 32°, ox freezing point of Fahrenheit; and 

 few will bear a higher temperature than 160 or 

 160, although instances have been adduced of 

 plants vegetating in situations where the tem- 

 perature, in consequence of volcanic action and 

 boiling springs, was not less than 212. The 

 points we have indicated, however, maybe looked 

 upon as the extremes of vegetable existence. It 

 is true plants, like animals, can bear a much 

 greater degree of cold than that of 82°; but any 

 temperature, much under this, suspends their 

 vegetative powers for the time, which are only 

 resumed after an elevation of temperature where 

 the juices of the soil on which they live are in 

 such a fluid state aa to be capable of being ab- 

 sorbed by their roots. 



The influence of heat, too, has a mai-ked ef- 

 fect on the development of the different parts of 

 plants; and thus leaves, blossoms, and fruit make 

 their appearance in successive periods or seasons. 

 From this circumstance, LinniEus consti-ucted 

 his Calendar of Flora, which comprehends a 

 view of the successive periods at which plants 

 blossom and produce fruit. With regard to the 

 frondescence, it must be evident to all that plants 

 do not produce their leaves at the same time. 

 Thus, the honey-suckle protrudes them in the 

 month of January; the goosebeiTy, currant, and 

 elder, in the end of February, or beginning of 

 March; the willow, elm, and hme tree, in April; 

 and the oak and ash, which are always the latest 

 among trees, in the beginning or towax'ds the 

 middle of May. Many annuals do not come up 

 till after the summer solstice ; and many mosses 

 not till after the commencement 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 it requires a greater or less degree of it to give the 

 proper stimulus to the vital energies of the plants. 

 But agreat many circumstances will always 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 pei-- 

 iod of advancement; and because the leafing of 

 the plant depends upon the peculiar degree of 

 temperature, and not upon the return of a par- 

 ticular day of the year. Hence it has been 

 thought that no rule could be so good for direct- 

 ing 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 cor- 

 respond best to each sort of grain respectively 

 in the degxee of temperature required. 



Linnseus, who instituted some observations 

 on the subject, about the j'ear 1750, with a view 

 chiefly to ascertain the time proper for the sow- 

 ing of barley in Sweden, regarded the leafing 

 of the beech tree as being the best indication 

 for that grain, and recommended the institution 

 of similar observations with regard to other ^rts 

 of grain, upon the gi'ound of its great impor- 

 tance to the husbandman. But however plausi- 

 ble the rule thus suggested, may be in appear- 

 ance, and however pleasing it may be in contem- 

 plation, it is not likely that it will ever be much 

 attended to by the husbandman, because nature 

 has furnished him with indications that are still 

 more obvious in the very evidence of his o^vn 

 feelings, as well as perhaps more correct; as all 

 trees of the same species do not come into leaf 

 precisely at the same time, and as the weather 

 may change even after the most propitious indi- 

 cations. 



The flowering of the plant, like the leafing, 

 seems to depend upon the degree of temperature 

 induced by the returning spring, as the flowers 

 are also protruded pretty regularly at the same 

 successive periods of the season. The mezereon 

 and snow drop blossom in Febmary, the prim- 

 rose in the month of March, the cowslip in 

 April, the great majority of plants in the months 

 of May and June ; many in July, August, and 

 September; some not tiU the month of October, 

 as the meadow safii'on; and a few not even till 

 winter, as the arbutus and laurestinus. Such 

 at least is the period of their flowering in this 

 country; but in warmer climates they are earlier, 

 and in colder regions later. 



In tropical countries, where the temperature 

 is steadilj' at a high elevation, it often happens 

 that plants will blossom more than once in the 

 year, because they do no more require to wait 

 till the temperature is raised to a certain height, 

 but merely till the development of their parts 

 can be effected on the regular operations of nature 

 under a temperature already sufiGcient. For the 

 greater part, however, they flower during our 

 summer, though plants in opposite hemispheres, 

 flower in opposite seasons. In all climates, 

 however, the time of flowering depends also 

 much on the elevation of the place above the 

 level of the sea, as well as on other causes affect- 



