PARASITIC AND SAPROPHYTIC PLANTS. 



37 



PARASITIC AND SAPROPHYTIC PLANTS. 

 By Rev. Professor G. Henslow, M.A., F.L.S., V.M.H., &c. 



[Lecture delivered on March 20, 1906. j 



General Observatioi^s. — Parasitic and saprophytic plants are distributed 

 over many orders of flowering plants as well as genera of fungi. In the 

 division Thalamiflorce of Dicotyledons there is only one ; in Gamopetalce, 

 seven ; in Incoinpletce, five ; while in Monocotyledons there are four.* 



Their most obvious feature, in a large proportion at least, is the total 

 absence of any green colouring matter or chlorophyll. All such are, 

 therefore, incapable of assimilating purely inorganic food materials, and 

 must depend wholly upon living plants as hosts, or decayed organic matter 

 in the soil. It was supposed, however, in this latter case, which applies 

 especially to Saprophytes, that their nutrition depended on the aid of a 

 fungus mycelium, which covers or penetrates the roots and supplies the 

 plant with nitrogenous substances. Such at least appeared to be the most 

 probable interpretation of this curious condition of symbiosis. At the 

 present day such fungi are more truly parasitic, as will be described. 



Since it is presumable, on the score of structural aflSnity, that greenless 

 parasites and saprophytes have descended from green plants, it is not 

 surprising to find that many such are parasitic, although they can derive 

 noui'ishment to some limited extent by their ordinary roots and leaves. 

 As an example, species of Pyrola or Winter Green, among the heath 

 family, supply us with a transition through the leafless species P. aphylla 

 to the order Monotropacece, which some botanists separate from EricacecB 

 because of their totally greenless condition. It includes four genera, one of 

 which, Monotropa Hypopitys, is British. Again, the order Scrophularinece, 

 which contains numerous non-parasitic genera, as the Foxglove, Snap- 

 dragon, Musk, &c., has a tribe, Euplirasiece, containing eighteen genera, 

 six of which are British, and all but one have green leaves. They are 

 also all parasitic, and on grass roots. From this tribe one passes to the 

 order OrobanchacecB or Broom Rapes, which are entirely greenless. 



Numerous parasites exist in the order Fungi of cryptogams, while all 

 the rest are saprophytes, none whatever having any chlorophyll at all. 



Flowering parasites attach themselves by penetrating root- hairs to 

 their "host" plants, or by means of suckers called haustoria ; these are 

 disc-like elevations produced by the superficial tissues, from the centre of 

 which the root-like process arises, which penetrates the stem of the host, 

 and then grafts itself upon the living tissues beneath the surface of the 

 latter. 



* In Dicotyledons there are the following orders :— Poly gale cb, EricacecB, Monotro- 

 pacecB, LenjioacecB, Gentianece, Convolvulacece, Scrophidavinece, OrobaiidiacecB, 

 LauracecB, Loranthacece, Santalacecs, Balajiojpliorec?, and Cytinacecs. In Mono- 

 cotyledons Melaiithacecz, Triuridece, Bwmanniacece, Orchidea are orders containing 

 plants which appear to be saprophytes rather than true parasites. 



