ORGANS AND SYSTEMS OF ORGANS. 159 



The main root, tap-root, characterized as the direct elongation of 

 the stem, is often destroyed and its functional activity is taken up 

 by the lateral roots. In plants developed from tubers and bulbs 

 (many monocotyledonous plants) there is no main root. 



Ordinary roots take up water and soluble substances contained 

 in the soil (see Physiology of Tissues, II, B), and serve to attach 

 the plant firmly to the soil. In warm moist climates many plants 

 possess aerial roots whose physiological importance we have already 

 learned to know. In certain cases (Pandanus, for example) these 

 aerial roots may enter the soil and serve as organs of support ; they 

 may even form the only support for the stem. In other tropical 

 plants the branches send out aerial roots which elongate and form 

 supporting organs (mangrove trees ; JOHOW). 



(b) Modifications of the Phyllome. 



The leaf-organ also presents various physiological forms or 

 modifications. The observer soon learns to distinguish germ-leaves 

 (cotyledons), cataphyllary (scaly) leaves, foliage-leaves, hypsophyllary 

 leaves, and floral leaves. Before discussing these in particular we 

 shall consider briefly the general morphology of the phyllome. 



In the highest type the leaf may be divided into three morpho- 

 logical parts : leaf-sheath, petiole, and blade (vagina, petiolus et 

 lamina) (Fig. 97). 



If, however, only two of the parts mentioned were present, it 

 would be wrong to speak of it as an undeveloped or imperfect leaf; 

 there are, for example, leaves consisting only of the 

 sheath-portion, as the bud-scales, bulb-scales, and rhi- 

 zome leaves; these are nevertheless highly perfect. 

 In those cases where one or the other of the parts 

 mentioned is absent it is because it would be useless; 

 this makes the part that is present so much more 

 important from a physiological standpoint. In the 

 discussion of the mechanical tissue-system we inci- 

 dentally mentioned the mechanical function of the 

 leaf-sheath. The sheath is the expanded basal por- 

 tion at the base of the petiole or at the base of the 

 blade; it encloses the stem. The stipules are special modifications 

 of the leaf-sheath. Example: Asperula odorata ; of the six or 

 eight leaf-like structures arranged in a whorl two are true leaves 



