564 ESTIMATION OF THE HEAT NECESSARY TO GROWTH. 



wanting. It can only be stated with certainty that the increase or diminution 

 of heat does not take any part in this remarkable periodicity. It would, however, 

 be going too far to assert of all species that the resting period, normally observed 

 by them, could not be shortened by external influences, especially by rise of 

 temperature. Many seeds, such as those of cress, mustard, barley, and numerous 

 so-called weeds, which appear as unwelcome guests on cultivated land, have no 

 resting stage, germinate at any season if they are supplied with the necessary 

 moisture; and the warmer the soil the more rapid is their development. It is 

 also well enough known that there are plants which, to use the language of gar- 

 deners, may be "forced". Tulips, Lilies of the Valley, and Lilac, whose resting 

 period lasts in Central Europe from the ripening time of the seed in summer until 

 the spring of the next year, may be forced even late in autumn if planted in a 

 greenhouse in warm, damp soil, soon after they have ripened their seeds and 

 have begun to rest. Under these circumstances they produce their flowers even 

 in January, and in these plants, consequently, the materials manufactured in the 

 previous summer may be used as constructive materials for growth almost at 

 once. I remember once drawing the shoot of a Clematis plant rooted in the open, 

 after it had lost its foliage in the autumn, through a narrow crevice 3 metres above 

 the soil into the interior of a neighbouring hothouse. Leafy shoots were developed 

 from the buds of the upper portion thus warmed even in December; while the 

 lower portion of the same plant, situated outside the hothouse and surrounded by 

 cold air, was still frozen. In this plant also, the materials manufactured in the 

 summer could be used as constructive materials as soon as ever they had been 

 deposited in the reserve storehouses. 



The same must indeed be the case in those plants which bloom normally in the 

 spring, but yet often in years characterized by particularly mild autumns, burst 

 open in October; the buds destined for the next spring thus sending out fresh leafy 

 shoots and blossoms twice in the same year — for example, many apples and horse- 

 chestnuts, violets and strawberries, many primulas, gentians, and anemones. 



Although it may be doubted whether the constants hitherto computed can be 

 taken as an accurate expression of the heat consumed by plants for growth in their 

 various stages of development, nevertheless their value must not be under-estimated. 

 Comparisons of results obtained in various places by the same methods, with the 

 same instruments, and on the same species, will without doubt yet lead to many 

 interesting conclusions. The determination of the commencement of the various 

 phenomena of development, the determination of the unfolding of the foliage and 

 flowM^s, of the ripening of the fruit, and of the autumnal leaf -fall — at as many 

 stations of observations as possible — is not only a highly attractive problem in it- 

 self, but is also of great scientiflc value; and this no less in its bearing upon the life 

 of plants generally, than upon the geography of plants, since the barriers which con- 

 fine plants in their distribution can be in great part explained by the fact that the 

 species in question are unable to complete their annual cycle of development on the 

 further side of the boundary. Finally, also for climatology, since the yearly process 



