I goo- 1.] Effhcts ok Watkr on Foliage Lkaves. 2^3 



the leaves remained alkaline, the chlorophyll was of a darker green 

 colour. Twenty-four hours produced less effect than sixteen. He 

 experimented with Elodea and Utricularia in a solution of ammonium 

 carbonate (one tenth of one per cent, strength) in water. Darwin, 

 in 1872, noticed that a precipitate in the cell-sap was produced 

 on treatment with dilute alkalies. This shows that the alkali 

 penetrates both the cell-wall and the lining of the protoplasm; 

 only after prolonged exposure were the cells killed. This precipitate 

 consists of tannin and other substances excreted by the ammonium 

 carbonate. 



Among the tobacco growers of America there are recognized two 

 diseases, or two forms of the same disease, of the tobacco plant, the 

 " calico " and the " mottled head." " Mottled head " is a condition in 

 which only the uppermost leaves are affected, and uhere the spotting 

 occurs at a somewhat later stage in the life of the plant. When the 

 plants are affected early in life, the middle and the lower leaves have the 

 characteristic appearance. This is known as "calico." About fifteen 

 years ago there was observed a peculiar mottled condition of the tobacco 

 plant in Holland. Dr. Mayer made some investigations upon this 

 condition and gave to it the name " Mosaic Disease." He concluded 

 that the disease was a bacterial one, but his investigations were not 

 carried on far enough to warrant his conclusion. He proved that a 

 sound plant might be inoculated with infected sap ; but that one plant 

 could not infect a neighbouring plant. 



In some early works giving a general description of the tobacco 

 plant, we find that one character given is that the plant on reaching its 

 maturity in a natural condition is quite likely to have light coloured 

 spots on its leaves. These spots were in no sense looked upon as a 

 disease, or as being at all hurtful to the plant. The leaves of the 

 suckers from healthy plants or stumps are often found spotted (Sturgis. 

 1898), and this would tend to show that this feature was not so much a 

 disease as a condition of the plant depending upon soil and atmospheric 

 conditions, or was philogenetic in its nature. It has also been shown 

 by Otto Carl Butterweck that one means the farmer has of knowing 

 when the tobacco is ready to cut is that the leaves begin to turn lighter 

 green, and light coloured spots appear upon them. This is doubtless 

 the time when the plant ceases to draw nourishment from the air and 

 soil, and is now concentrating its nourishment that was scattered 

 through all parts. According to Dunal on Solanaceai (1852), some 

 species of this genus, Nicotiana, have peculiar grey-spotted, or a dirt\- 



