PARASITISM OF CUSCUTA. 



27 



Like the mistletoe, so also are the species of Cuscula parasitic on the aerial 

 green shoots of woody plants : their parasitism is, however, complete, since they not 

 only possess no roots fastening them into the soil, but they also completely lack 

 chlorophyll, and are necessitated to take the whole of their nourishment from the 

 host. This they do by means of hausioria, which arise within the twining stem of 

 the Cuscuta only where this closely surrounds the host-plant, and it is apparently 

 the pressure hereby exercised which induces the origin of the haustoria. That these 

 latter are to be regarded as reduced roots can hardly be doubtful from all the 

 researches before us, as well as from our figure. Not only the place and the occa- 

 sion of their origin, but also the primary young stages of these organs, correspond 

 with those of typical roots. In the further development, however, a striking deviation 

 from these appears. The tissue of the hausiorium, corresponding to the body of the 



FIG. i6. — Haxistoyiifin oi Cuscitta e^thnum on the axial vascufar bundle [g), beneath the cortex 

 {rr) of the shoot-axis of the Cusciita, ee epidermis of latter ; E epidennis of Linum stem ; 

 R its cortex ; /^ its wood. The axis of the Ctiscitta and its hattstoriitm in longitudinal section ; 

 stem of LinHtn in transverse section (magn.) 



root, becomes arranged into a bundle of rows of elongated cells, which, growing 

 forward at the end, break through first the cortex of the Cuscuta, and then the cortex 

 of the host-shoot, to penetrate to the woody body of the latter, or even to break 

 through into its pith. Hereupon, indeed, the cell-rows of the body of the root 

 may become isolated as separate threads, which grow into the tissues of the host. 

 In the axis of the hausiorium, the axial cord of vascular bundles is also still to be 

 recognised, and the vessels of this become fused with the vessels in the wood 

 of the host-plant. This intimate union of the hausiorium of the Cuscuia with the 

 stem of the host-plant becomes aided yet more by an outgrowth of the tissue of 

 the Cuscuta, which surrounds the hausiorium in the form of an annular wall, and 

 becomes closely applied to the shoot of the host. 



Similar haustoria, departing still further from the type of true roots, become 

 developed on the otherwise typical roots, branching in the earth, of Thesium and 



