PRECIPITATION AND VEGETATION 37 



the dying down of herbaceous plants is due to the fact that the 

 temperature in autumn and winter is low. 



The botanist would doubtless consider that this statement lacks 

 the accuracy which comes from a full physiological treatment, 

 but from our standpoint the essence of the matter is that in our 

 climate the vegetative rhythm is determined by temperature, and 

 not by seasonal differences in amount of moisture. But in winter, 

 when outside all is bleak and bare, our rooms are gay with bulbous 

 plants, which in the summer of our warm rooms or greenhouses 

 will flower luxuriantly in the midst of snow. Now, as we all 

 know, the activity of these winter-flowering bulbs is of very short 

 duration : they flower, they ripen their bulbs, as the gardeners 

 say, and they die down. But it is clear that the Freesia or Ixia, 

 or the more familiar hyacinth, which sleeps throughout our 

 summer, has not its life rhythm punctuated by changes of tem- 

 perature as most of our native plants have. Such bulbs actually 

 pass the warmest part of our year in the resting condition. How 

 can we explain this anomaly ? If we try to find out in what 

 countries the greatest numbers of bulbous plants occur naturally, we 

 find that they occur specially in the Mediterranean region, 

 in South Africa, in Southern California, and so on, that is, in 

 countries where the rain only falls in the winter season. 



Take the Mediterranean region as nearest to us. Here there 

 is no temperature check to vegetation as with us ; generally speak- 

 ing there is warmth enough for vegetative activity throughout 

 the year. But in the summer-time, when the sun is hottest, 

 there is no rain, therefore the plants are obliged to take advant- 

 age of the winter rains for their flowering. They rush through 

 their activities in a very short space of time, and it is water, not 

 rising temperature, which wakens them. Those of the plants 

 of the Mediterranean which are not bulbous have usually 

 small leathery leaves ; they are often prickly ; generally they 

 show the desert type of structure. We notice from watching 

 cactuses in the greenhouse or Botanic Garden that again it is 

 moisture which induces flowering, not a rise of temperature. 

 This subject might of course be much further elaborated, but 

 these few hints may be useful to the teacher in planning lessons 

 on rainfall and vegetation. 



