DIV. I 



MORPHOLOGY 



110 



The leaves and stems 

 LEAF-SCARS mark the 



FIG. IBl.Helleborusfoetidus. Foliage leaf (I) 

 and intermediate forms between this and 

 the bract (A). (Reduced. After SCHENCK.) 



subterranean shoots, decay while still attached, 

 of the aerial shoots of herbs die off together, 

 places where the fallen leaves were 

 attached to the stem. Plants in which 

 the foliage leaves remain active for 

 several seasons are called EVERGREEN 

 in contrast to DECIDUOUS forms. 



The fall of the leaves in phanerogamic 

 woody plants is effected by means of a 

 parenchymatous ABSCISS LAYER which is 

 formed at the base of the leaf-stalk shortly 

 before the leaf is shed. In this region all 

 the mechanical tissu%s of the petiole are 

 greatly reduced, only the vessels being 

 lignified. The separation of the leaf results 

 from the rounding off of the cells of the 

 absciss layer, the middle lamellae becoming 

 mucilaginous, while the vessels and sieve- 

 tubes are broken through. The protection 

 of the leaf -scar is effected by the cells 

 exposed by the wound becoming transformed 

 into a lignified cutis tissue and, later, by 



the formation of a layer of cork produced from a cork cambium and continuous 

 with that covering the stem. 



D. The Branching of the Shoot ( 56 > 66 ). The more foliage 

 leaves that can be exposed to the sunlight on a shoot the greater 

 will be the amount of organic substance formed by assimilation. In 



this respect, as will be evi- 



c d dent, a branched system of 



shoots is greatly superior 

 to a single erect shoot. 

 The former can expose leaf- 

 surfaces to the full sunlight 

 over a much greater area. 



As in thalloid plants 

 the branching of the shoot 

 can happen in two ways. 

 Rarely the shoot forks, 



FIG. 138. -Rhizome of rolygonatum vudtiflorum. a, Bud of dividing into two daughter 

 next year's aerial shoot ; ft, scar of this year's, and c, d, e, G 



scars of three preceding years' aerial shoots ; w, roots. ax 6S (DICHOTOMY). Usually 



(1 nat. size. After SCHENCK.) the branching is LATERAL, 



the daughter axes being 

 thus formed on the main axis which continues its growth. 



A. Diehotomous Branching. This is confined to the shoots of 

 some Lycopodiaceae. 



In such Club-Mosses, when a shoot is about to divide into two equal branches, 



