DROUGHT EFFECTS 59 



order may be changed since the injured member loses water 

 more rapidly than any other part of the plant and, as a re- 

 sult, is almost certain to die first. If a leaf has been injured 

 by having the epidermis broken by the force of the wind or 

 by the impact of hail, if it has been punctured by sucking 

 insects or lacerated by biting insects, it commences to die 

 from the point of injury, as soon as the plant fails to obtain 

 sufficient water to keep its tissues turgid. The stem may be 

 similarly affected by such injuries. The root is often in- 

 jured by insects, and even more frequently by the filaments 

 of parasitic fungi. Plants suffering from root injuries are 

 almost certain to die from the root upwards. 



When the plant is healthy and vigorous, and has not re- 

 ceived any mechanical injuries, the growing point in stems 

 and rosettes is the last part lio die, because the meristem 

 cells hold their vitality more tenaciously than other cells 

 and are able to live to the last. In succulent plants they 

 even seem to be able to draw upon the neighboring cells for 

 water. This may be the case in all plants. "When the period 

 of drought is prolonged but not intense, the roots of fibrous- 

 rooted plants commonly die before the stem, but after all or 

 most of the leaves have perished. This is the way many 

 monocyclic herbs die ; Atriplex argentea, Phaseolus vulgaris, 

 Euphorbia marginata, Ambrosia artemisiifolia are good ex- 

 amples; Phaseolus, Coleus, Oaura, Ambrosia and a number 

 of other plants partially adapt themselves to slow drought 

 by dropping their leaves, the oldest first. In this way they 

 check rapid evaporation and soon reduce it to a minimum. 



Dicyclic herbs often have more or less fleshy roots which 

 hold water tenaciously in times of drought ; especially is 

 this the case when the plant is in the rosette stage. The 

 leaves during this period of the plant's existence die much 

 sooner than the roots. 



When a herbaceous plant sends up a stem preparatory 

 to flowering, the growing point of the stem holds water in 

 its calls tenaciously, and where the plant has not suffered 

 from physical inj ury, the leaves die first, the root next, 

 then gradually the stem dries out and last of all the young 



