BACTERIAL ACTION ON THE PLANT. 69 



TISSUES ATTACKED. 



In what we may consider as the lowest type of these diseases the parenchyma-cells of 

 storage tissues are the parts principally attacked. Often these are aggregated in fleshy 

 organs which have reached maturity and ceased to grow but abound in water, amid, proteid 

 and carbohydrate substances, designed for the green plant which is to be developed the 

 coming season. Buds, bulbs, tubers, rhizomes, and various swollen underground parts of 

 mixed structure are good examples. Examples of such diseases are certain soft-rots of 

 potato-tubers, Jones's carrot-rot, Metcalf's rot of sugar beet, and the coconut bud-rot 

 (plate 5, and figs. 19 and 20). Usually they do not appear in green, growing parts. They 

 destroy the tissues by softening the middle lamellae. In a little higher grade of essentially 

 the same type of disease, the green parts of plants are also attacked, e. g., iris-rhizome-rot, 

 Appel's potato-rot, lettuce-rot, calla-lily-rot. All these organisms require tissues rich in 

 water, otherwise they refuse to grow or make very little headway. 



A grade higher in the scale, perhaps, are those bacteria which attack the parenchyma 

 of stems, roots, bark, green leaves, etc., but can do so only when the tissues are in a rapidly 

 growing, actively dividing condition. They may retain a foothold for some time thereafter, 

 in exceptional cases, but their power for evil is limited to a short period of the growing 

 season. Black-spot of the plum and pear-blight (vol. I, plates 28, 29) are good examples. 

 All of the leaf-spots are primarily diseases of the parenchyma, and some of them are limited 

 to quite restricted areas of the parenchyma, e. g., leaf -spot of the carnation, larkspur, soy- 

 bean. Other diseases like pear-blight and Aderhold's cherry-blight often extend a long 

 distance through parenchymatic tissues. In case of the pear the upward or downward 

 movement of the bacteria in the bark may be several meters, and the sidewise movement is 

 often sufficient to girdle and kill large limbs or even the whole tree. 



In general, however, destruction is more extensive when the organism is able to attack 

 the vascular system as well as the parenchyma. Then we have phenomena of occlusion, 

 and marked interference with transportation of water. There are intermediate forms and 

 transitions of various sorts as might be expected. The pear-blight organism, so far as I 

 know, seldom follows the vessels, expending its energy rather on the cortical parenchyma. 

 Some leaf-spot bacteria seem to use the vessels more than others. 



Appel's potato-rot organism (B. phytophthorus) makes some use of the vessels of the 

 stem, but seems more at home in the parenchyma. On the contrary the organism causing 

 Stewart's sweet-corn disease, to take a very striking example, develops principally in the 

 vascular system and destroys the plant from this vantage ground. This is true also of 

 Bacillus tracheiphilus and Bacterium vascularum. The same is true of Bad. solanacearum, 

 in potato and tomato, only here the organism finally floods out into the tissues of pith and 

 bark much more than in the other cases cited (fig. i ) . Bad. solanacearum and Bacillus 

 phytophthoms may be compared and contrasted in this particular since both attack the 

 potato. Both make use of the vessels, but the former does so much more extensively and 

 destructively than the latter; the one is primarily a vascular disease, the other a paren- 

 chyma disease; one destroys the stem by occluding the vessels, the other by rotting it off 

 at the surface of the earth. All vascular diseases make pockets in the parenchyma, but 

 in most cases these are only in close proximity to the vessels (fig. 6) and after the latter have 

 been occluded and destroyed. In very soft tissues such as those of watery, fast-growing 

 tomato shoots, Bact. solanacearum finally makes very extensive closed cavities, often honey- 

 combing both pith and bark for many centimeters. Bacterium 'vascularum, though restricted 

 pretty closely to the bundles for long distances in the maturer parts of the stem of sugar- 

 cane, often excavates extensive closed cavities in the very soft undeveloped parenchyma 

 under the terminal bud. The gum diseases rupture the bark and ooze extensively on the 

 surface. Pear-blight does this also to a lesser degree. Some, perhaps all, of this class of 

 bacteria reach the surface through fissures due to surface tensions set up in dead tissues by 



