212 DISEASES OF PLANTS 



It is not easy to see why or how the influence of the para- 

 site on blooming can be so direct as it appears to be. 



Smuts, of which corn-smut is probably the most famil- 

 iar, cause a great deal of damage among cultivated plants. 

 The blackness or smuttiness is due to the formation of 

 black one-celled reproductive bodies (spores) which are 

 thrust out through the epidermis of the infected part. 



Bacterial Infection of plants is now recognized to be 

 the cause of certain common maladies, some of which are 

 of great economic importance. Thus pear blight, a bac- 

 terial infection of the inner bark, causes the flowers 

 to abort so that the quantity of fruit formed may be very 

 seriously reduced. Again, so-called wilt in cucumbers, 

 etc., may completely destroy a valuable crop. This dis- 

 ease is due to the infection of the water-conducting tissues 

 by a certain species of bacteria. Alfalfa and other plants 

 develop at the top of the root or base of the stem enlarge- 

 ments known as crown-galls. These also are due to 

 bacterial infection and, because of the nature of the tissue 

 growths following such infections, comparisons have been 

 made between these growths and their infecting cause 

 with cancer in the human body. It cannot be denied 

 that there are certain resemblances in the behavior of the 

 cells of infected tissue in crown-galls with the growths 

 taking place in cancer. 



Root Tubercles. — The roots of leguminous plants, 

 such as peas, beans, clovers, and alfalfa, generally form 

 enlargements known as tubercles or nodules. Examina- 

 tion of these shows that they also are the seat of bacterial 

 infection. The infection takes place through the root- 

 hairs; the infecting bacteria traversing the root-hair and 

 penetrating into the outer portion of the root produce 

 such an irritation that an outgrowth, in some cases resem- 

 bling a lateral root, is formed. This consists mainly of 

 thin-wallod cells in which the infecting bacteria multiply 

 very rapidly. The formation of these tubercles is seen, 

 therefore, to be the result of infection. It is a departure 



