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 induced. The want of light causes the true chlorophyll colouring matter to remain in abeyance, and consequently the plant as a whole suffers from carbohydrate 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.