PHANEROGAMIC PARASITES 67 



disease attacking a single branch of a tree. I consider the 

 effect to be the outcome of some physiological disturbance, 

 resulting in the production of a ferment capable of dissolving 

 the pectic substances present in the middle lamella, and thus 

 allowing the cells to break away from each other. 



'Silver-leaf is widely distributed. M'Indoe states: 

 ' European varieties of plums are not so extensively grown in 

 Auckland as in Otago, where the climate is cooler. The 

 trees, however, suffer so much from silver-leaf disease that 

 they are passing out of cultivation, gages being the only kinds 

 doing well.' 



M'Indoe, Gard. Chron., March 21, 1909. 

 Percival, Journ. Linn. Soc^ 35, p. 390 (1901-4.). 



PHANEROGAMIC PARASITES 



In Britain we have but few of the higher plants that affect 

 a parasitic habit, and some amongst these do very little injury 

 to cultivated plants. The Broomrapes (Orobanche) are our 

 most pronounced parasites, and, having no trace of chloro- 

 phyll, depend entirely on the host-plant for food from the 

 moment of germination of the seed. The very minute seeds 

 do not germinate unless in contact with the root of a suitable 

 host-plant, and almost immediately after germination become 

 attached to a root, from which they obtain the food necessary 

 for their development. The dodders (Cuscutd) come next in 

 the order of parasitic evolution. The seeds, which contain a 

 small amount of reserve material, germinate in the ground 

 independent of the proximity of a suitable host-plant. The 

 radicle or tiny root enters the ground, but, as in the broom- 

 rapes, is not provided with root-hairs, and simply absorbs a 

 certain amount of moisture from the ground, the tiny thread- 

 like stem growing to a length of one or two inches at the 

 expense of the reserve material in the seed. When this is 

 exhausted the plant continues to grow for some time, drawing 

 on the supply of food contained in the swollen radicular 

 portion of the seedling. During this period the slender stem 

 slowly undergoes the movements of circular nutation, feeling 

 as it were for the stem of a host-plant. If no such plant is 

 forthcoming the seedling dodder perishes when the supply of 

 food contained in the lower portion of the plant is used up. 



