i8o DISEASE IN PLANTS. 



Etiolation is due to insufficient intensity of light, 

 the pale sickly yellow organs being unusually watery 

 and deficient in vascular tissue, the internodes 

 abnormally long and thin, and the leaves generally 

 reduced in size, or, in some plants also " drawn." 



Forced Endive, Rhubarb, Asparagus, and earthed 

 Celery afford examples of etiolation purposely in- 

 duced. The want of light causes the true chloro- 

 phyll colouring matter to remain in abeyance, and 

 consequently the plant as a whole suffers from car- 

 bohydrate starvation. 



Laying of Wheat and other cereals is a particular 

 case of etiolation. The seeds having been sown 

 too thickly, the bases of the haulms, owing to the 

 etiolation and consequent lack of carbohydrates, 

 suffer from want of stiffening tissues, and the top- 

 heavy plants fall over. 



False etiolation depends on a similar abeyance 

 of the chlorophyll, but in this case due to too low 

 a temperature. It is often seen in Wheat and 

 other monocotyledons when the young leaves 

 unfold in cold weather in spring. The symptoms 

 of "drawing" and tenderness are however absent. 



Pallor due to too intense illumination must be 

 kept sharply distinct from etiolation, the pale green 

 or yellow hue being here due to the destruction of 

 the chlorophyll by insolation, and the accessory 

 symptoms of " drawing " are wanting. 



Chlorosis is a form of pallor where the chloro- 

 phyll grains themselves are fully developed, but 

 their green pigment remains in abeyance owing to 

 a deficiency of iron in the soil, and can often be 



