192 



BOTANY OF THE LIVING PLANT 



is no self-nutritive function necessary in the parasite, and the leaves 

 which normally carry it on are not required. Such reduction of the 

 vegetative system is usual in complete parasites. But this reduction 

 does not apply to the floral region. The flowers of the Dodder, which 

 are produced in dense heads, arise each in the axil of a bract, and struc- 

 turally they show the characteristic features of the Convolvulaceae. 

 From these facts the conclusion seems justified that Dodder is a type 

 related to Convolvulus, and that the twining habit has led to its 

 parasitism. Whereas Convolvulus is dependent by twining only for 

 mechanical support, Dodder has gone a step further, and has become 

 dependent upon its host for its complete physiological support also. 





Fig. 143. 



Cuscuta europaea. Section vertically through a sucker, which projects from the stem of the 



Dodder, shown above in the drawing, into the stem of the host. See Text. ( x 35.) 



Further, as it is an isolated parasitic genus, and its flowers are like 

 those of Convolvulus, though on a smaller scale, it appears probable 

 that its parasitic habit has been acquired relatively late in its evolution. 

 The attachment of the Dodder to its host is by means of suckers, 

 which probably represent highly modified roots. The details of the 

 connection appear to be variable in different species, and perhaps on 

 different hosts. In specimens of Cuscuta europaea the facts appear 

 strongly to support their root-character (Fig. 143). First an adhesive 

 disc projects from the stem of the parasite, and becomes closely 

 appressed to the surface of the host, attaching itself by rhizoid-like 

 hairs. Endogenous tissues then burst through Hke a root, and pene- 

 trate the tissues of the host. The superficial cells of the penetrating 

 sucker then grow out into tubes of varying length. Some of these 

 apply themselves to the wood, others to the bast, others to the pith 



