156 ON THE POWER OF MOVEMENT IN PLANTS. 



movements of leaves, but probably has to do with the contractile 

 power of protoplasm in the attached parts of their organs, and 

 with variation in turgescence. 



Moisture has to do with some floral movements in what are 

 called Hygroscopic plants, and so we can predict meteorological 

 changes often with tolerable accuracy. The Siberian Sow-thistle 

 closes at night if the following day is going to be fine, and the 

 reverse. Bindweed, Marigold, and Pimpernel, if open, close on 

 the approach of rain. Many leaves are hygrometric, as are also 

 some larger Algce^ giving similar indications. Of these flowers, 

 Pimpernel is most reliable, keeping its petals closed if clouds are 

 coming or come ; open if a fine clear day is at hand. This flower 

 is called ' Poor Man's Weather-glass.' It is not only a very eco- 

 nomical barometer, but a very sure one, which is more than we 

 can say of some of those costing ;^20, or so ; if Pimpernel be 

 wide open at 9 a.m., you may leave coat and umbrella in your hall 

 with almost perfect safety. 



The time of year at which flowers open has to do with Jjitefisity 

 of light, and fairly comes under our present head ; we are so accus- 

 tomed to regard it as a matter of course, that Daffodil opens in 

 March or April, Rose in June, Borage in July, Tansy in August, and 

 so on, that we are apt to lose sight of the ' why ' of the facts. The 

 * why ' lies in the amount of light needed to stir their opening 

 mechanism, the amount needed for them to do their work, and the 

 amount they can bear, so to speak, as also of heat. Plants forming 

 their floral-buds in the Autumn, and opening next Spring, are 

 usually short-flowered ; those budding and expanding in the same 

 year usually remain open much longer. 



So it is with Geographical differences. A plant opening at 6 a.m. 

 in Senegal does not open here until 8 or 9 a.m., and in Sweden 

 not until 10 a.m. One opening in Senegal at 10, opens here at 

 noon, and not at all in Sweden ; one opening at noon in Senegal 

 will not bloom either here or in Sweden ! So plants existing, say 

 at Smyrna, Berlin, and Stockholm, flower in February, April, and 

 June respectively. Along these same lines are the 'forced' plants, 

 as we call them, acted on by artificially increased light and heat, 

 as in the cases of Hot-house and Stove plants, and those grown in 

 electric light. 



