88 PLANT PHYSIOLOGY 



Gaston Bonnier* claims that Viscum is sometimes directly 

 beneficial to its host. In summer the host produces both 

 actually and proportionally more food than the parasite, 

 although the parasite is then photosynthetically active. At 

 times during the winter and always during the early spring, 

 when the host is leafless, the evergreen parasite may manu- 

 facture carbo-hydrates and the host cannot. While the host 

 is not able to manufacture food and the parasite is, the 

 host is alleged to draw upon the mistletoe for freshly manu- 

 factured non-nitrogenous food. It is, therefore, claimed by 

 Bonnier that the mistletoe ( Viscum ) is parasitic either not 

 at all or only very slightly, although obviously it obtains 

 all its water and mineral salts from the host. But because 

 the parenchyma tissues of the parasite are continuous 

 through the haustoria with those of the host, furnishing the 

 paths of osmotic transfer, the transfer taking place in one 

 direction at one time may be in the opposite direction at 

 another; and so it must be conceded, until proof to the 

 contrary is adduced, that the mistletoe may be more than 

 a "water parasite." Why should the host, on the warm 

 days of winter and in the early spring, have any occasion 

 to draw food from the mistletoe? A healthy apple-tree, or 

 oak, or poplar, will normally lay by in its own body during 

 the summer enough elaborated food to start it well in the 

 succeeding spring. Upon this store it will draw promptly 

 and satisfyingly when the need comes. Is there any less 

 food stored in an apple-tree upon which mistletoe grows? 

 If so, is not the mistletoe the cause, and can this lack ever 

 be wholly compensated for? It would appear, then, that 

 Viscum is a periodic rather than a partial parasite, and 

 that only further investigation can show how nearly its 

 indebtedness to its host is annually balanced. 



Closer association with the host und greater dependence 

 upon it are exhibited by the various species of Cuscuta or 

 dodder, thread-like plants belonging to the Convolvulacese, 



* Bonnier, G. Assimilation du Gui comparee a celle du pommier. Actes du 

 Congres de 1889 d. 1. Societe Bot. de France : Bull. Soc. Bot. de France, 

 1890. Sur 1' assimilation des plantes parasites a chlorophylle. Comptes 

 Rendus, t. 113, p. 1074-6. 



