Structural Botany : Perennation. XII. 



Special adaptations to enable organisms to exist in a more or less dormant 

 condition over periods of unfavourable environment are included as phenomena of 

 Perennation; since such vicissitudes are most commonly presented in terms of 

 seasonal periodicity. Conspicuous examples are ( i ) the case of the Cold Northern 

 Winter, (2) that of the hot and dry Southern Summer. Both trace back to problems 

 of water-supply, rather than temperature which only affects the rate of metabolism : 

 e. g. in cold soil, mechanism of root-absorption fails, and plants perish for want of 

 water in cold wind, even if rooted in water. In hot deserts lack of water is more 

 insistent than extreme insolation as leading to xerophytic habit. In cold winter of 

 British Isles trees perennate from mid-Oct.-Nov. to April-May, vegetating in short 

 summer season of 6 months ; and leaf-fall becomes a conspicuous feature of the zone 

 of deciduous trees (Central and N. Europe), as opposed to highly specialized xerophytic 

 foliage of evergreens of the Mediterranean region. 



Mechanism of Leaf-fall, a special case of the general method of cutting off 

 all useless parts by the formation of an absciss -layer ; a tract of cells whose walls 

 readily exaggerate intercellular spaces, to loosen entirely along a line of cleavage ; all 

 parts intentionally separated follow this method, whether leaf, flower, fruit, or seed. 

 Autumnal leaf-fall may be more elaborate, as commonly preceded by a formation of 

 phellogen across the leaf-base (except through V.B.), giving a cork-layer in connexion 

 with the cork of the stem, and thus ' healing the wound ' before it is made. The 

 cork-layer becomes uniformly continuous, except across the axillary bud. 



Populus (Poplar), leafy twigs cut in early October show the cork-zone with 

 phellogen on inner side, in usual radial rows ; V.B. pass through it : all tissues on 

 the leaf side are dead, and full of cluster-crystals; but a trace of starch at the 

 extreme base. On the stem side all living units are gorged with starch (as also calc. 

 ox., and patches of stone-cells, as conspicuous details). The absciss-layer is formed 

 externally to the first-formed cork (often giving phloroglucin-reaction) ; when formed, 

 the leaf loosens, hence it is not seen fully developed in sp. mat. Mechanical action 

 of wind or frost separates the dead leaf, rupturing the vessels and fibres of the V.B, 

 The V.B. are subsequently plugged by outgrowth of bundle-parenchyma, as a wart- 

 like scar on each end. The surface of the entire plant is then sealed with a uniform 

 zone of cork. 



The Winter Bud : Such deciduous trees protect the shoot-apex and young 

 growth by special formation of" bud-scales (leaf-base, or stipules) ; and in order to start 

 rapid growth in spring, the leaves of the succeeding season are commonly all laid 

 down in the previous summer, remaining in a resting-stage enclosed within the ' bud ', 

 with its investment of scales (hibernaculum). The finest winter bud is that of 



Aesculus (Horse-chestnut), not indigenous ; buds cut in October show Bark 

 with Lenticels, Leaf-scars with smaller scars of V.B., 5-7 in an arc; large T-bud, and 

 smaller axillary laterals following the decussate arrangement of the leaves. The 

 series of bud-scales similarly decussate imbricately in 4 rows, longer distally to the 

 pointed apex, and sticky with resinous exudation (sol. in spirit) from special glands 

 (colleters) on the scales. 



In longit. med. sect, note the length of successive scales ; the enclosed foliage- 

 members with cottony packing-hairs; and in finer shoots the central terminal 

 inflorescence of young buds. Section of bud-scales shows colleters as small wart- 

 like sessile glands, with secretion poured into chinks between the scales. The latter 

 develop cork on outer surface from hypodermal phellogen. 



Similar buds in April show rapid extension of shoot-internodes, divergence of the 

 bud-scales ; and often interesting transitions in form from protective leaf-base scale to 

 distal palmately lobed lamina. 



Winter-buds are wholly wanting in trees of warm climates ; just as leaves remain 

 on more than one season as ' evergreen' habit. But leaf-fall may be again induced by hot 

 dry season (desert vegetation), resting buds similarly perennating over heat-period. 



Herbaceous Perennials commonly vegetate over winter by a ' root-stock ' 

 portion, at the soil-level or below, of stem parts with reduced internodal extension ; but 

 sending up rapidly elongating axes each season, as foliage and flowering-shoots ; cf. 

 Polygonum, Hop. 



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