PATHOLOGY 75 



Also of the nature of an accident are the various 

 forms of poisoning that may happen to healthy plants. 

 They may be poisoned by gases in the atmosphere, or 

 they may be poisoned by minerals in the soil. In the 

 cases of slow poisoning the growth of the tissues may be 

 arrested or altered and truly pathological conditions 

 set in, in which abnormal cell growths take place. On 

 the other hand, where the poison is stronger, the plants 

 simply die, as, for instance, when the paths are sprinkled 

 with weed-destroying compounds. These enter the 

 roots in the osmotic process of root absorption, and 

 travel through the cells of the tissues. 



Accidents may happen to the healthiest individuals ; 

 the pathologist is more concerned with the diseased 

 ones and with those where the tissues are abnormal. 

 One of the most fatal diseases that can overtake a 

 plant is Chlorosis, or the lack of colouring matter. This 

 disease, in its essentials, is very similar to anaemia in 

 human beings, and as the plants depend on their colour- 

 ing matter for the manufacture of their own food, an 

 extreme case cannot survive at all. Chlorosis is an 

 obscure disease, but in some cases it certainly appears to 

 be caused by a lack of iron, and without iron the human 

 blood is not red nor plant granules green. Generally 

 the seedlings attacked by the disease die out very early, 

 but sometimes sickly whitish-leaved specimens struggle 

 along for a little while. The disease is often local, and 

 in compound leaves one leaflet here and there may be 

 entirely colourless. This character is best seen in the 

 gardeners' u variegated " varieties, where the leaves 

 are mottled or striped with cream-coloured patches 

 and bands. The green parts there do enough work to 

 carry on the life of the individual, while the colourless 

 parts are non-producers. If this is not carried too far 



