Of Aphyllon Uniflonim, Gray. 1 1 5 



Structure of the Stem. 



In cross section the stem shows an epidermis of small 

 flattened cells, thick-walled on the outer side (Plate XIV, 

 Fig. 3). Beneath is the cortex, composed of large, rounded 

 cells ; there are usually from eight to ten layers of these cells, 

 although the stems vaiy considerably in this respect. The 

 cortical cells are closely packed with large rounded starch 

 grains. Within is a more or less continuous ring of degen- 

 erate bundles, widely separated in some places by medullary 

 rays. Within the bundle-ring again is the pith, its large 

 rounded cells packed with starch grains. 



The bundles are arranged in the usual manner, with exter- 

 nal phloem and internal xylem. A reduced bundle-sheath is 

 present as a frequently interrupted ring of small rounded cells. 

 The phloem consists of elongated elements with granular 

 contents, which are sometimes nucleated. As in the root, 

 sieve-tubes seem to be entirely absent. The protoxylem con- 

 sists of one or two spiral tracheae with cells. The secondary 

 xylem consists of short pitted-reticulate tracheae, strongly 

 indurated. A cambium is not generally present, although 

 some stems show interrupted lines of small cells which may 

 doubtfully be interpreted as cambium. Although the xylem 

 elements are fairly numerous in root and stem, it seems prob- 

 able that the wood has nothing to do with the conduction of 

 nutritive liquids, but serves solely to support and strengthen 

 the plant. 



The presence of such quantities of starch in a colorless 

 parasitic plant is somewhat perplexing. The starch cannot 

 have been brought over from the host as such, since starch 

 is insoluble, so that leucoplasts must be present, and in con- 

 siderable numbers. They are, however, small and difficult to 

 make out clearly. Moreover it is puzzling to see such an 

 amount of reserve food stored in all parts of a purely annual 



