HORTICULTURAL JOURNAL. 329 



other times expanded, contract or fall together, because they lose their na- 

 tural elasticity and turgescence, which is occasioned by the action of the 

 sun's rays. The sleep of plants is in no respect to be compared with that 

 of animals. 



The barometer indicates the changes of weather, even before they are 

 preceptible to our senses. We say that it forebodes them. In that respect 

 also do certain plants. Not only do many flowers close up when it rains, 

 but there are many plants which forebode rain, in the same sense as the 

 barometer forebodes the changes of weather. Calendula pluvialis closes 

 its flowers, and Porliera hygrometrica its leaves, when rain is about to fall. 

 Warmth and dry air are necessary, which cause them to expand and evapo- 

 rate strongly, to retain their natural size. The dampness of a cloudy or 

 misty atmosphere, which precedes rain, causes the parts to fall together; 

 this and the sleep of plants are, therefore, called hygrometric phenomena. 



When the evaporation is disturbed, the plant becomes sick, notwithstand- 

 ing the supply of nutritious particles by the root ; this has appeared from 

 observation and experiment. If the leaves are smeared with varnish, or 

 any other glutinous matter which hinders the evaporation, then the plant 

 dies, for the surface can no longer be active. 



In July, 1845, there appeared, to an alarming degree, in one of the 

 plants most generally used for food (the Potato) a disease probably not un- 

 known formerly. Fields, which till lately had grown luxuriantly, showed 

 only death and decomposition ; the hope of the farmer on a large crop dis- 

 appeared. Many people forboded bad harvests for following years, and ad- 

 vised a change in the plan of agriculture. An extraordinary hot summer 

 weather preceded, followed by a sudden falling of the thermometer, having 

 showers, and cold days. The plant, which had grown too luxuriantly, was 

 at once disturbed in its growth and in its evaporation. The liquids were 

 forced to remain in the plant ; they had no means of escape, the tubercle 

 was not ripe — the fcecula not yet formed, but, floating in an over-abundant 

 water, stood still of necessity, and soon rotted ; the leaves and stem withered 

 and became rotten. That is, it appears to me, the simple, primary cause, 

 and the course of a sickness, which was a national calamity here and else- 

 where, and the sad consequences of which we see more or less every year. 

 The leaves are to be compared to the lungs and skins of animals ; they 

 remove from the body substances which could not, without damage to the 

 organism, remain in the mass. But while they draw the saps upwards, 

 they are nourished by them, and borrow from the fluids raised the substan- 

 ces necessary for their own development. At first white, thin, membranous, 



