44 



LECTURE IV. 



developed shoot-axis possesses inter-foliar parts, as is usually the case, these are 

 usually clothed with a well-developed epidermis, which is in its turn covered by 

 a cuticle and perforated by stomata. Hairs also of the most varied form, prickly,^ 

 woolly, or glandular, are very common appendages. If the typical leaf-shoots are 

 destined to last several or many years, their epidermis becomes sooner or later 

 replaced by a uniform cork layer, a so-called periderm, which carries on the mam 

 function of the epidermis (viz. the prevention of the loss of water by evaporation), 

 more completely than the epidermis itself. 



s—] 



w 



FIG. 37.— Median longitudinal section through a suinil 

 winter bud of Abies pectiitata. ss scale-like enveloping 

 leaves, which arise from a cup-like outgrowth of the 

 cortex i.'vr) of the preceding year ; z pith of the new 

 rudimentary shoot ; v its growing point 



FIG. 38. — Median longitudinal section through a vigorous 

 winter bud of the Horse-chestnut [j^isciihts hippocastaniini^ 

 m, h, r pith, wood, and cortex of shoot-axis (T/y) of the pre- 

 ceding year ; ss bud scales ; // young foliage-leaves J in the 

 middle is the young inflorescence— the dotted space is filled 

 with woolly hairs. 



In the interior, the young shoot-axes consist of a basis of succulent par- 

 enchymatous tissue, in which run a few or very many vascular bundles, often also 

 sclerenchymatous cords. While in the roots the vascular bundles are united into a 

 single axial strand, surrounded by the parenchymatous cortex of the root, the vascular 

 bundles of the shoot-axes on the other hand run, primarily at least, as isolated 

 filaments, the upper ends of which curve out into the leaves, while their lower 

 ends attach themselves to the middle parts of the preceding bundles. Only in 

 rarer cases, e.g. in some water plants (Hippuris) and a few Cryptogams {Marsilea, 

 Pilularia, &c.), is the shoot-axis traversed by an axial strand of vascular- 

 bundles, a so-called cauline bundle, to which the bundles of the leaves became, 

 attached. Thus, in contrast to the roots, each single vascular bundle of the shoot-': 

 axis is surrounded by parenchymatous ground tissue. If the shoot-axes, however, 

 form true wood later on, which in fact only occurs in the Coniferse and Dicoty-i' 

 ledons, then a so-called Cambium-layer arises, which is developed in part in the 



