252 ABSORBING SYSTEM 



are specially adapted for saprophytic nutrition. 131 In the genus 

 Buxbaumia (B. aphylla, B. indusiata), the protonenia is green and 

 capable of photosynthesis ; but the bulbous stem, and the leaves with 

 which it is clothed, are entirely devoid of chlorophyll. There is, on 

 the other hand, an extraordinarily well-developed system of rhizoids, 

 which differ from typical Moss-rhizoids in having quite thin colourless 

 cell-walls. Another remarkable feature is the not uncommon occur- 

 rence of fusions between separate rhizoid branches, which lead to the 

 production of H -shaped connections, and here and there even to the 

 formation of a regular network of rhizoids. Altogether the rhizoidal 

 system of Buxbaumia strikingly resembles an ordinary Fungus- 

 mycelium. 



The rhizoids of Rhynehostcgium muralc often penetrate into the 

 dead stems and roots of Vascular Plants ; in so doing, they do not 

 undergo any great change of form, except that they are always more or 

 less constricted where they pass through the substance of a cell-wall. 

 In another member of the Hypnaceae, Earhynchium praelongum, 

 which generally grows among rotting leaves, the rhizoids perforate 

 the outer walls of the epidermis, and then pass from one epidermal 

 cell to another by boring through the lateral walls. These rhizoids 

 develop very conspicuous lobed outgrowths, which accommodate them- 

 selves to the contours of the epidermal cells, and sometimes completely 

 fill their cavities (Fig. 99 a). Where such rhizoids happen to penetrate 

 into elements of the mesophyll, they show the same tendency to occupy 

 the entire cell-cavity. 



A very interesting specialisation has been observed by the author 

 in the case of a variety of Webera nutans, which grows upon moist, 

 rotten Fir wood. In this instance the rhizoids on penetrating the 

 substance of the wood grow mainly along the walls of the tracheides ; 

 here and there, however, a rhizoid puts forth short processes which 

 perforate the thick walls of the tracheides just like the hyphae of a 

 parasitic Fungus (Fig. 99 p.). These processes usually arise from 

 irregular, rounded or lobed expansions of the rhizoids ; their thickness 

 is from one-third to one-sixth of the average diameter of the parent 

 rhizoid. After a process has passed through the wall of a tracheide, 

 its tip once more expands, and thenceforward grows on as an ordinary 

 lateral branch. 



Phanerogamic parasites 132 abstract nourishment from their hosts in 

 a variety of ways. The development of their absorbing system is 

 naturally very largely dependent upon the extent of their parasitism. 

 Where the parasite retains possession of functional foliage leaves, the 

 haustorial absorbing tissue does not expose a very large surface, and is 

 altogether comparatively uuspecialised. Frequently, as in the case of 



