442 ZONES AND REGIONS [Pt. Ill, Sect. II 



commencement of the true flowering season usually coincides with that 

 of the vegetative season in spring and ends with the cessation of this 

 season in autumn. The beginning and end of the flowering season are 

 dependent to a great extent on the temperature, and therefore not only 

 fail to coincide at different places, but also in different years at the same 

 place. But the action of the inherent rhythm always proves stronger than 

 that of temperature, so that the awakening of vegetation very often occurs 

 at a lower temperature than does its falling asleep. 



It has been already shown \ that in the temperate zones low tempera- 

 tures usually favour the inception of reproductive organs, and that the 

 growth of these organs is completed in very many instances within lower 

 limits of temperature, or at any rate possesses a lower optimum than is 

 the case with foliage-shoots. The dissimilar action of temperature on the 

 reproductive and vegetative organs respectively has not only been proved 

 experimental!}', but is also apparent in nature, especially in lower crypto- 

 gams and mosses, the sexual activity of which very often coincides with 

 winter, whilst their vegetative activity occurs during the warm months. 

 In phanerogams thi.-; connexion is less clear, owing to a number of opposing 

 circumstances. Thus, many herbaceous plants can commence forming 

 flowers only after they have assimilated the needful material ; the formation 

 of assimilating organs however, like assimilation itself demands higher 

 temperatures. Other plants again are adapted to certain pollinators and 

 flower at a time when these are active. In spite of such limitations, the 

 favourable influence of low temperatures on the reproductive organs in 

 relation to the periodicity of phanerogams is unmistakable when we 

 consider those plants alone that produce the plastic materials for the 

 manufacture of flowers in the preceding vegetative season. 



It is well known that, in the temperate zones, most herbaceous perennials 

 belonging to the Liliaceae, Amaryllidaceae, Iridaceae are early-flowering ; 

 in Mediterranean countries Orchidaceae and Araceae also are among the 

 earliest spring-flowering plants. Man}' Dicotyledones with rhizomes or 

 roots rich in nutriment, such as Anemone, Helleborus, Eranthis, Corydalis, 

 Ficaria, behave similarly. A few bulbous plant.s flower in autumn, such 

 as Colchicum autumnalc, Spiranthes autumnalis, Crocus sativus, Cyclamen 

 europaeum. The majority of trees are also earl}'-flowering and frequently 

 open their flowers earlier, and therefore at lower temperatures, than their 

 leaves. The indigenous examples, with which the ivy as a late-flowering 

 plant is included, are sufficiently well known. I had noticed the same 

 feature in the much richer tree-flora of North America, and an analysis 

 of the flowering times of the trees in Asa Gray's Manual of the Botany 

 of the Northern United States has confirmed my observations. Of 141 

 species referred to in the above work as being trees or arborescent shrubs— 



' See p. 4S. 



