DISEASES OF PLANTS. 137 



the Dodder germinates in the soil exactly as a clover seed germ- 

 inates ; but when the delicate stem begin-s to cling to the stalks of 

 other and stronger plants it is making arrangements to live upon 

 their food. The Dodder soon sends forth roots which insinuate 

 their way into the tissues of the victim and there absorb the sap 

 which it regards as its legitimate plunder. The parasite first 

 strangles the plants on which it climbs, and then, having lessened 

 the demand for sap, takes it itself, adding insult to injury. When 

 its roots are well fastened in the tissues of its neighbor the ground 

 root is no longer of use, and hence the connection with the soil 

 ceases. In flax-growing countries this plant is of great injury to 

 the stems of flax, holding them in a firm grasp, living upon their 

 life. Both the mistletoe and the broomrape are plants of this class, 

 but they are not so widely disseminated nor so destructive to the 

 plants on which they live, as the parasite just described. 



The injurious effects produced by animalcules and insects are to 

 be treated in this class of diseases produced by mechanical 

 injuries. The wheat-eel or vibrio tritici, is the cause of a disease 

 termed ear-cockle. The late Professor Henslow of Cambridge 

 University, attentively studied this affection, and gave the 

 results of his investigations in the Journal of the Royal Agri- 

 cultural Society of England, volume second. Just one century 

 before Professor Henslow began his investigations, Needham had 

 examined under the microscope the blighted grains of wheat 

 affected by the vibrio. " The grains which are thus infected turn 

 dark green at first, and ultimately nearly black ; and they become 

 rounded somewhat, resembling a small peppercorn, but with one 

 or more deep furrows on their surface. The husks of the chaff" 

 spread open, and the awns are twisted, by which means the 

 infected ears are readily observable among the standing corn. 

 Upon opening the blighted grains, they are found to be filled with 

 a moist cottony substance ; but to contain no flour. When Needham 

 placed this cottony mass in a drop of water, under his microscope, 

 he perceived to his surprise, that it was composed of a multitude 

 of minute eel-shaped animalcules, which were in active movement, 

 twisting and wiggling to and fro, like so many eels and snakes." 

 Remember that these observations were made one hundred and 

 twenty-five years ago. Since the time of Needham numerous 

 students have confirmed his results. "When a sound grain of 

 wheat is sown by the side of one infested with the vibrio, the 

 young plant which springs from the former is not infected," until 



