ESTIMATION OF THE HEAT NECESSARY TO GROWTH. 563 



moistening the surrounding soil, so that we might have the Snowdrop blossoming 

 even in November. But very many experiments have shown that bulbs so 

 treated, although they develop leaves and an inflorescence, do not properly 

 develop their flowers, and always perish prematurely; while four months later 

 the growth of the leaves and flowers takes place easily and quickly at tempera- 

 tures which are not much above zero. And in many root-stocks, in most buds of 

 branches above the ground, and in numerous seeds and spores, the same thing 

 occurs as in tubers and bulbs of which the Potato and Snowdrop have been selected 

 as well-known examples. How many plants there are which blossom early in the 

 spring, ripen their fruits in the early summer, and whose seeds, being detached from 

 the parent plant in the height of summer, lie scattered on the ground! Although 

 the soil in which they are imbedded is damp and sufficiently warm, and although all 

 the external conditions of germination are fulfilled, yet they do not germinate in the 

 same year in which they have been produced. Not until the following spring do 

 the embryos put forth their rootlets, and then usually under conditions apparently 

 much less favourable than those of the previous summer and autumn. These seeds 

 are not yet ripe, or rather, perhaps, they are not yet capable of germination when 

 they fall from the parent plants. The materials contained in their cells must first 

 pass through a process of transformation before they can promote the development 

 of the embryo, and this transforming process can in no wise be hastened by in- 

 creased supplies of heat and moisture. In many large seeds, as, for example, those 

 of hazel, beech, and almond, this difference between seeds just fallen from the tree 

 not yet capable of germination, and the seeds which have been mellowed and can 

 germinate, may be easily perceived in their consistency, taste, and smell. The 

 phenomenon here described is found in a specially remarkable manner in the fruits 

 of the Water Chestnut (Trapa natans). If in autumn water-chestnuts just ripe be 

 placed in water, and the temperature of the water be kept through the winter at 

 15 C., the rootlets of the embryos will not emerge until the coming spring, and not 

 then on account of a higher temperature, but at the same temperature to which they 

 have been continuously exposed for six months. If the temperature of the water be 

 raised to 20 the growth of the rootlets is not accelerated, and the increased tempera- 

 ture cannot become effective as an incitement to growth until after the seeds 

 have been suitably prepared for a period of six months. Gardeners say that such 

 seeds must "mellow" and "ripen after gathering", and they have indeed hit the 

 mark with this latter expression. Some spores also must mellow and ripen for 

 a much longer time. Many, of course, germinate immediately after their detach- 

 ment from the parent plant. The so-called resting spores, however, always pass 

 through a quiescent period, whose duration usually can be determined with great 

 accuracy and may be shortened a little by altered external influences. Very 

 remarkable is the fact that in the seas of tropical regions, whose waters possess 

 the same chemical composition, temperature, and illumination from one year's end 

 to the other, certain species of red seaweeds develop in March, others in June, and 

 others again in October. In these instances all grounds for an explanation are 



