HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 115 



cut off on the outer side of the meristem, c and d by cells 

 cut from the inner side. 



In Sunflower, which is typical of Dicotyledons in gene- 

 ral, c and d are distinct, the " penblem " being traceable 

 to a single layer of cells covering the apex of the "plerome" 

 and being itself covered by a layer which gives rise to the 

 root-cap and the piliferous layer. 



In Maize, however, the piliferous layer when traced 

 towards the apex is seen to be continuous with the " peri- 

 blem," so that the root-cap tissue alone is developed 

 towards the outer side of the apical meristem, the layer 

 from which it arises being termed the " calyptrogen." 



135. Aerial Root of Tropical Epiphytic Orchid. In trans- 

 verse sections of the aerial root of a tropical epiphytic Orchid (e.g. 

 Oncidium, Vanda, Dendrobium), note (a) the vascular cylinder 

 with its alternating xylem and phloem bundles, pericycle, endo- 

 dermis strongly thickened but with passage-cells opposite the 

 xylems ; (6) the cortex, consisting of rounded cells containing 

 chloroplasts ; (c) the exodermis, a layer of cells mostly with 

 thickened walls, but some thin-walled and forming passage-cells ; 

 (d) the velamen, consisting of several layers of transparent empty 

 cells which serve to absorb arid store water. 



If fresh material is available, note that the aerial root appears 

 white when dry (the velamen then containing only air) and green 

 when moist (the velamen being then transparent and making the 

 green colour of the cortex visible). 



In tangential sections, note the fibrous thickenings on the walls 

 of the velamen-cells. 



136. Haustorium of Dodder. Get material of plants (Gorse, 

 Heather, etc. ) infested with Dodder, and with scissors cut it into 

 pieces which can be held in pith so that sections may be cut passing 

 through both plants at the places where the Dodder stem is attached 

 to its host ; arrange that some sections shall cut the Dodder stem 

 transversely, others longitudinally. 



Note that the Dodder stem is fixed to the host stem by a disk, 

 the superficial cells of which are often greatly enlarged, and that 

 from this disk a haustorium has grown into the tissue of the host 

 stem. Some of the haustoria will be seen applied to the vascular 

 bundles of the host. Note that each haustorium has a central 

 xylem strand with spiral vessels, continuous with the bundles of 

 the Dodder stem. In favourable sections this strand may be traced 

 right into the xylem of the host plant, while the haustorium has 

 also elements which join on to the phloem of the host. 



Similar, but smaller, attachment organs and haustoria can be 



