92 BOTANY PART i 



the leaf surfaces can be placed horizontally and obtain favourable illumination. 

 This is frequently attained by twisting of the internodes, which thus transforms 

 a decussate into a two-rowed arrangement on inclined shoots. Similar changes 

 occur in the case of alternately- arranged .leaves in relation to the best utilisation 

 of the light by the leaf surfaces. The position of the foliage leaves is nearly 

 always an adaptation to the needs of the plant as regards light. In some 

 horizontal subterranean shoots (e.g. of Ferns) the leaves stand in one row on the 

 upper side. 



Practically nothing is known of the causes of the regularity in the arrangement 

 of leaves. The assumption of SCHWENDENER that purely mechanical causes 

 acting at the places of origin of the leaves determined the arrangement of the 



latter has proved to be unfounded ( 57 ). 

 The leaves need not arise at the apex in 

 the order of their genetic spiral, nor 

 simultaneously as members of a whorl. 

 Sometimes one side of a growing point 

 may even predominate in the production 

 of leaf-rudiments. 



B. Primary Internal Struc- 

 ture of the Stem ( 58 ). The stem 

 exhibits a much more advanced 

 differentiation of tissues than the 

 long shoots of even the most 

 highly segmented thalli. On the 

 outside a typical EPIDERMIS forms 

 its boundary layer. Beneath this 

 in the internodes (the nodes have 



PIG. 109. Transverse section of an internode of a more Complicated structure to 



the stem of Zea Mais, pr, Primary cortex ; pc, foe Considered later) COmCS a ZOUC 

 pericycle ; cv, vascular bundles ; ere. funda- ,. . ,. r i -> -i i 



mental tissue of the central cylinder, (x 2. of tissue free from Vascular bundles 



After SCHENCK.) and called the CORTEX. This sur- 



rounds the CENTRAL CYLINDER 



(Fig. 109), as the remaining tissue of the stem including the vascular 

 bundles is called. 



It is practically desirable to maintain the conception of a central cylinder even 

 though in some Monocotyledons the cortex cannot be clearly distinguished from 

 the central cylinder and the vascular bundles occur close below the epidermis. 



Cortex. The cortex is mainly composed of parenchyma. In 

 green aerial shoots the peripheral layers contain chlorophyll, while 

 those farther in are colourless and serve for storage rather than 

 assimilation. In colourless subterranean stems, which often attain a 

 greater thickness, it is composed of colourless parenchyma which, like 

 the parenchyma of other regions of the rhizome, contains reserve 

 materials. Mechanical tissue is also developed in the cortex. The 

 stem in aerial shoots sustains the weight of the leaves and is exposed 

 to bending by the wind ; it must be sufficiently rigid against bending 

 in all directions. This is attained by the aid of mechanical tissue as 



