deep ; but often very wide and very deep (broad rays of Quercus). In wood split 

 down radial planes the rays show as a ' silver-grain ' effect. 



IV. Cork: The epidermal cells constitute a specialized integumentary system 

 producing the cuticle, but are not capable of indefinite growth and division. Typically 

 in woody stems with rapid secondary growth, the epidermis is replaced by a new 

 formation (Periderm). A special meristem arising from the cells below the epidermis 

 (in the general case hypodermal, but often deeper) presents the dividing mechanism of 

 a cambium, so far as ' initial cells ' and ' radial rows ' may be concerned. This 

 constitutes the Phellogen ; and the tissues formed from it centripetally are cork, those 

 centrifugally phelloderm : the latter is rarely well- developed, and merely adds new units 

 to the cortex, only distinguished by being in radial rows. 



The cork-cells retain close lateral contact, but soon die ; the wall is ' suberized ', 

 and becomes very impermeable to water and gases. The cork-formation may be 

 continued indefinitely from the same phellogen (cf. Bottle-cork), or new phellogens 

 may be initiated internal to the first one. All parts external to the last phellogen are 

 cut off from communication with the cortex, and soon die, decay, or may be exfoliated. 



V. Bark : The term bark botanically covers all such dead layers cut off by 

 a Phellogen, whatever their origin, as opposed to the popular and commercial concep- 

 tion of the word. Such bark may be cut off in flakes or rings, sheets or ' scales ', the 

 last by oblique sectorial phellogens, and may contain ' stone-cells ' (Pintts) and layers 

 of different texture. Where the phellogens are initiated deep in secondary phloem, 

 the bulk of the bark of an old tree may consist largely of crushed phloem. The 

 products of one phellogen are traced in terms of radial rows to each initial. 



VI. Lenticels : As the cork in its special impermeability replaces cuticle in 

 function, so aeration of the tissues must be provided for, and the stomatal control is 

 replaced by lenticel-formation. These appear on corky surfaces as small wart-like 

 growths, often increasing with age to considerable dimensions on the bark, but are of 

 essentially similar origin from the phellogen : certain cells over localized areas are set 

 apart to give new tissue-units centripetally, differing from cork- cells in that the walls 

 readily separate, and the tissue becomes hygroscopic and powdery, as ' lenticel tissue ' : 

 on decay, this leaves free communication to the phellogen and subjacent phelloderm, 

 which is usually well-developed beneath the lenticel. Such lenticels may persist for 

 several years, and show seasonal periodicity ; closing in winter by a production of 

 cork, and opening in spring as the cork-layers are in turn ruptured, and thrown back 

 like the pages of a book, by new loose-tissue formation. Clogging or closing of these 

 pores by dust or smoke, by cutting off the free oxygen-supply from the internal 

 cambium, where there is little photosynthesis in the cortex, may be distinctly injurious. 



Elder (Sambucus) shoots cut at the end of the first summer show a well-defined 

 production of large cork-cells (50 /x, wide), in radial rows of half a dozen or so from 

 hypodermal phellogen ; the dead epidermis still persisting, and not conformable with 

 the subjacent radial rows. Phelloderm represented by 1-2 cells per radial row; the 

 initial cell of the Phellogen as the last cell out with protoplasmic contents. Under- 

 neath the Periderm collenchymatous cortex. The woody ring is extremely hard, and 

 may be avoided ; slips of outer cortex, stripped at the cambium, being cut in pith. 

 Lenticels show as lenticular areas, about 2-3 mm. long, and i mm. broad ; in section, 

 note great extension of dead layers, ruptured peripherally to a broad gap; loose 

 lenticel-tissue, and great development of phelloderm, a dozen cells or more in the 

 radial rows. Larger growths on second year's shoots, to 2-3 mm. diam., show 

 characteristic reflexed tissue-zones. 



Bottle Cork (Quercus Suber), as a secondary growth of cork, induced after 

 stripping primary mass ('Virgin Cork*'), of very uniform texture, with lenticels as 

 conspicuous pore-canals. These pass radially and transversely, giving the orientation 

 of the tissue. 



Note annual zones of growth (2 mm. wide) ; in transv. sect, radial rows of fairly 

 isodiametric rectangular units, about 40 /x, wide ; radial sect, similar radial rows ; but 

 in tangent, sect, no radial rows are seen ; the cell being isodiametric and polygonal. 



Tilia (Lime-tree) is a convenient type of woody stem, owing to softness of wood. 

 (Separate detailed Schedule omitted from elementary course) 



18 



