AND THE DISEASES OF PLANTS. 295 



They arise from certain juices of plants, which, from some 

 unknown cause, change their nature, and thus affect the 

 color of the leaf. These changes are preserved if the 

 plants are multiplied by subdivision, and even sometimes 

 continued when propagated by seed. 



The second class of diseases results from circumstances 

 connected with the undue supply of elements, which are 

 in themselves necessary to vegetation ; such as tempera- 

 ture, light, water, air, soil, &,c. If a plant has too much 

 or too little light, heat, or water, it has no means of avoid- 

 ing the excess, or of compensating for the deficiency. 



Caroline. The poor plant, it is true, rooted to the 

 ground, cannot, like an animal, fly the evil, or seek a rem- 

 edy; it must patiently submit to it, and endure the diseases 

 it entails : if the soil afford too much nourishment, it must 

 continue feeding, and cannot stop when its appetite is 

 palled, 



Emily. Or, what is worse, and more frequently the 

 case, when the soil does not yield a sufficiency of nourish- 

 ment, it cannot seek it elsewhere, and famine must debil- 

 itate the" roots, and diminish that vigor which would ena- 

 ble them to stretch out their fibres over a greater extent 

 of soil. 



Mrs. B. We have already entered so much into de- 

 tail on the influence of light, heat, water, and soil, on 

 plants, that I shall confine myself to recalling a few of the 

 most essential points to your memory. Excess of light 

 produces too much excitement ; the oxygen escapes, and 

 the carbon is deposited too rapidly ; the plant vegetates 

 in a fever, and the sap, incapable of supplying its wants, 

 is exhausted ; the plant withers, and the leaves fall off. 

 There are two modes of remedying this disease ; either to 

 increase the aliment, or diminish the vegetation ; the first 

 may be done by plentiful watering, the other by dimin- 

 ishing the intensity of the light. 



Excess of heat dries up the juices ; if you attempt to 

 remedy this by plentiful watering, the plant sprouts leaves, 

 but very little fruit, 



Caroline. This sort ofvegetationmustbe well adapted to 

 meadows, where a produce of leaves is principally aimed at. 



1592. Of party colored leaves, what is said from what do they 

 arise'? 1593. Of the second class of diseases what is said? 1594. 

 How does excess of light afiect plants] 1595 .-^What are the modes 

 of remedying this disease'? 1596. If there is an excess of heat, what 

 is swicll 



