332 , TRANSACTIONS OF SECTION K. 



2. On the DeseH Flora of Western Egypt. 

 By Capl. H. Hamshaw Thomas. 



During the spring of 1916 some months -were spent in camp on the edge of 

 the Libyan desert, about 25 miles north west of Cairo, and this afforded some 

 opportunity of observing the flora of the region and the exceedingly imf avourable 

 climatic conditions under which the plants grow. The winter rains are light, 

 but in the spring very heavy dews occur at night, while during the day the 

 pebbly soil becomes exceedingly hot. Vast stretches of gravelly desert occur, 

 which, unlike the sandy and rocky desert of eastern Egypt and Sinai, are almost 

 totally devoid of vegetation. A few individuals manage to grow in sandy 

 pockets, while a small ephemeral Erodium succeeds in growing on the gravel. 



On the sandy desert edge a number of dwarf tufted plants with stout tap 

 roots occur. A specimen of Galligonium, comosum, was dug up but did not 

 appear to confirm Volkens' estimate as to the length of its subterranean root 

 system. 



Attention is drawn to the effect of the peculiar sand-storms known as the 

 ' Khamseen,' which occur in May and June just at the close of the growing 

 period of the vegetation. By an intensely dry and strong wind, together with 

 the eroding power of blowing sand, a large pxoportion of the younger shoots of 

 the plants are killed and broken off. The results of these storms are shown in 

 the dwarf, tufted habit of so many of the plants, especially in those lacking 

 stout thorny branches. 



3. Mycorfhiza and the Ericaoeee. By M. C. Raynee, D.Sc. 



Calluna vulgaris. 



Experimental proof has been published in an earlier paper that the relation 

 between this plant and its mycorrhizal fungus is an obligate one. 

 Tlie facts are as follow : 



(a) The fungus present in the root-mycorrhiza is distributed throughout the 

 vegetative tissues of the shoot, and is present in the ovaries of the flower. 



(b) Infection of the seed coats of the seeds takee place within the developing 

 fruit, and the seeds are shed carrying the fungus. 



(c) Infection of the new generation takes place at germination of the seed. 

 Unless such infection occurs, the seedlings are unable to form roots and quickly 

 perish. 



New facts are now presented bearing on the following : 



(1) The existence of a similar condition of 'obligate symbiosis' in other 

 members of the Ericacerp. The genus Vaccinium,. 



(2) The bearing of the facts already known on the edaphic peculiarities of 

 Calluna. 



(3) The biology of the relationship. Possibility, of nitrogen-fixation by the 

 fungus. 



4. The Morpholoyy of the Stele of Platyzoma microphyllum. R. Br. 

 By John McLean Thompson, M.A., D.Sc, F.L.S., F.R.S.E. 



In the materials of Platyzoma so far examined the stele was an unbroken 

 medullated cylinder devoid of leaf-gaps or perforations. It possessed inde- 

 pendent outer and inner endodermal sheaths iDstween wliich lay in centripetal 

 succession pericycle, outer phloem, xylem and inner parenchyma. The pith was 

 entirely isolated wnthin the conductive cylinder having no ground-tissue con- 

 nection with the cortex. In leaf-trace formation and departure the inner 

 endodermiis was in no way involved, and no point existed where the stele was 

 not entirely enclosed by an outer endodermis. 



It has been suggested that the pith of Platyzoma is the result of confluence 

 and final isolation of a series of foliar ' pockets,' and is in reality of extraetelar 

 origin, or is at least most reasonably interpreted as such, though, as a result of 

 stelar reduction, it no longer communicates with the cortex. The stele itself 

 has been considered reduced from an original solenostele by loss of internal 

 phloem and leaf -gaps As an alternative interpretation it has been suggested th^t 



