EXPERIMENTS ON THE ROOTS OF PLANTS. 517 



similar results. I am convinced that by causing artificial night for the 

 plants during the day* the excretion of the roots would be instantly 

 much increased ; but in all the plants that I have tried, I always 

 found that it continued slightly during the day. As it is well known 

 that by day the action of the light causes the roots of the plants to 

 absorb the liquid which contains their nourishment,* it is natural to 

 suppose that the absorption would cease during the night when the 

 excretion takes place. 



It appeared probable that by means of the roots the plants might 

 throw off the substances which they had imbibed, which were injurious 

 to vegetation. To satisfy myself on this point, and at the same time, as 

 the result was another means of verifying the existence of the excretion 

 of roots, I tried the following experiments : some plants of annual 

 mercury (Mercnrialis annua), carefully taken up, and washed with 

 great precaution in distilled water, were so placed that a portion of their 

 roots was plunged in a slight solution of acetate of lead, and the other 

 portion in pure water. They continued to live very well during several 

 days ; after which the pure water evidently precipitated the black 

 hydrosulphate of ammonia, and consequently had received a certain 

 quantity of salt of lead, rejected by the roots which were soaked in it. 

 Groundsel (Scnecio wilgaris), cabbages, and other plants, placed in the 

 same manner, produced the same results. Some plants, which were 

 placed in a slight solution of acetate of lead, lived very well during two 

 days, after which they were taken out. Their roots were washed in a 

 large quantity of distilled water, carefully dried, again washed in dis- 

 tilled water, which precipitated no hydrosulphate, after which they were 

 left to vegetate in rain water : in two days the reactives demonstrated in 

 the water a small quantity of acetate of lead. 



The experiments were made in lime water, which, being less hurtful 

 to vegetation than acetate of lead, was preferable for the object 

 sought after. When part of the roots were steeped in lime water, and 

 part in pure water, the plants lived very well, and the water consider- 

 ably whitened the oxalate of ammonia which demonstrated the presence 

 of lime. Also, a plant that had been kept in lime water, and washed 

 until the water no longer precipitated the oxalate of ammonia, then 

 transferred into pure water, after some time discharged a great quantity 

 of lime, which was demonstrated by the reactives. 



I repeated the same trials with a slight solution of sea salt, and the 

 nitrate of silver also demonstrated that the salt, which the plant had 

 imbibed by absorption, was partly ejected by the same roots which had 

 imprudently admitted it. 



