430 



cactus with a spot differentiated from its healthy surroundings by a scarcely 

 perceptible yellowish discoloration and a very slight convexity : e, indicates 

 the epidermis ; /, the collenchyma-like thickened cells ; o, the crystals of 

 calcium oxalate. The change begins close to the vessels (<7) in the delicate 

 venation traversing the succulent parenchyma. The darker spots in the 

 parenchyma indicate the chloroplasts, which are found there either in the 

 normal position along the walls, or collected in large refractive drops of 

 cell contents (o'). Probably as a result of an accumulation of destructive 

 enzymes and an increase in acid content, the sheath cells of the vascular 

 bundle {(js^ and those even further away {%) become poorer in contents and 

 elongate, thus causing the first evidences of disease. Thus an inner growth 

 is produced which, if it advances nearer to the upper surface, starts the for- 

 mation of cork. If the cells, extending further back into the inner bark, 

 become impoverished, more and more cork will be formed. Since the cork 

 tissue cannot elongate as the organ growls, it must be of necessity rupture, 

 and thus forms superficial warts as the cork formation advances. Grooves 

 are formed by the strain of the tissues growing with varying rapidity and 

 these deepen until there is a complete perforation as in deep scurvy of 

 potatoes. 



In order to control or eradicate this important disease of cacti, the 

 water supply is lessened and air is given abundantly. Should there be a 

 regular appearance of the disease covering several years, the plants must 

 be kept dry even to shrivelling. 



Bitten or Perforated Leaves. 



In herbaceous plants, as also in trees in different localities, the leaves 

 are often strongly perforated as if some animal had eaten away the tissue 

 betw^een the veins, without, however, finding any animal on whom the 

 blame may be laid. Since the injuiy increases in intensity with time, ob- 

 servers are more eager to find the cause. In extreme cases the injury is of 

 such extent that the leaves appear like many paned windows, since only the 

 network of veins remains together wnth a slight margin of leaf parenchyma. 

 .Such leaves are often bent and twisted but do not die prematurely. The 

 shoots themselves show no disease and frequently new sprouts with normal 

 foliage develop in the axils of these perforated leaves. 



The most extreme case which I have had opportunity to observe was 

 found in potatoes. The shoots of the plants at the beginning of July bore 

 only perforated leaves (see Fig. 72). Usually the lower leaves were per- 

 forated only in places, the upper ones were split longitudinally in the areas 

 between the veins and frequently parts of the edge were also destroyed. 

 The younger leaves often had a feathery appearance since the different 

 leaflets consisted only of the veins with a very slender margin. 



Between the perforations yellowish points were seen in the leaf-blade 

 when held to the light. These proved to be the first stages of the process of 

 suberization which ended with the perforation of the leaf. The formation 



