HAUSTORIAL ORGANS OF PARASITES 



255 



cells (Fig. 100 b) remind one forcibly of the scutellar epithelial cells of 

 Grasses. Each of the vascular strands of the haustorial process likewise 

 expands distally into a tuft of tracheidal elements, which place them- 

 selves in direct communication with the wood-vessels of the host. 



The above description of the haustorium of Thesium pratense 

 applies only to those cases in which the host-root pertains to a 

 Dicotyledonous plant. When the host is a Grass or other Monoco- 

 tyledon, both the haustorium as a whole, and the haustorial process 

 in particular, undergo a considerable amount of modification. The 

 haustoria of Thesium thus pos- 

 sess a certain power of accom- 

 modating themselves to different 

 host-roots. 



All the species of Cuscuta 

 (Convolvulaceae) are plants 

 with filiform twining stems, 

 which envelop the aerial organs 

 of their hosts in an inextricable 

 tangle of branches. They pos- 

 sess neither roots nor green 

 leaves. The entire supply of 

 food materials and water must 

 therefore be taken in through 

 the haustorial organs which are 

 borne on the stems ; these 



haUStOria are accordingly pro- A haustorium of Cuscuta europaea penetrating the stem 



virlprl with a lorrror QVionrhinrr oi Urtica ditrica (iatter in T.8.). The absorbing tubes are 

 Viueu W1GI1 a larger aUSOIOlllg most ahundant i y developed in the pith ; a few penetrate 



ciirfaoc. rv in nrliov TOmvlc nro ;ls f:lr ils tne hadrome strands, or even reach the leptome 

 Sliriace 01, 111 Otlier WOICIS, are after Dreaking through the woody cylinder. 



more highly specialised than 



the corresponding organs of Thesium. Such a haustorium of Cuscuta 

 consists of a circular or irregularly lobed adhesive disc (appressorium), 

 which is firmly anchored in the cortex of the host-stem by means of a 

 conical haustorial process. The tip of the process bears the actual 

 absorbing tissue, which consists of a fan- or pencil-shaped tuft of 

 elongated hair-like cells. In the case of G. epilinum some of these 

 tubular absorbing elements penetrate between the cortical cells of the 

 host, while others lay themselves against the woody cylinder, as a rule, 

 however, without breaking through the latter. The haustorial process 

 of G. europaea which most often grows upon the Xettle penetrates 

 still more deeply into the host-stem (Figs. 101 and 102), actually 

 breaking right through the woody cylinder, and developing its absorbing 

 tissue most abundantly within the pith. Some of these absorbing tubes 

 soon acquire the characteristics of tracheides and approach the oldest 



Fia. 101. 



