THE ANATOMY OF THE SHOOT 71 



proceed in the opposite direction, since the older portion of the 

 axis is that part of the hypocotyl adjacent to the cotyledonary 

 plate, and the tissues laid down distal to it are necessarily younger. 

 Supplementary intercalary growth of the hypocotyl also may serve 

 to complicate the description of transition in terms of progressive 

 development. 



The type of root-stem transition depends upon the character of 

 the root axis, whether it is diarch, tetrarch, or polyarch; the 

 character of the stem bundles, whether they are collateral or 

 bicollateral; and the nature of the seedling development, hypogeal 

 or epigeal. There are several well-recognized types of transition, 

 and many of them are described in detail in connection with specific 

 plants in later chapters. Regardless of type, the essential point 

 of transition is that a reorientation of primary vascular tissues 

 is effected so that vascular continuity may be established and 

 maintained. 



Sometimes actual continuity is attained only through the 

 development of secondary vascular structures, but usually they are 

 not involved. The cambial layer forms a lateral meristem which 

 is continuous through root, hypocotyl, and stem; and the 

 mechanism of secondary thickening is identical in all these regions. 

 For this reason, the secondary vascular tissues are not only con- 

 tinuous throughout the axial extent of the plant, but have the same 

 spatial relationships in the root, stem, and transition regions. 

 There is no reorientation of the epidermis, cortical parenchyma, 

 endodermis, and pericycle in the transition zone. 



GENERAL MORPHOLOGY OF THE LEAF 



The shoot has been defined as a stem axis with its divergent 

 members, lateral branches, and leaves of various types. The princi- 

 pal leaf categories are: foliage leaves, which perform the photo- 

 synthetic function; bracteal leaves, which are primarily protective; 

 scale leaves, or cataphylls, which may be vestigial or perform impor- 

 tant functions in storage and nutrition; and sporophylls, which con- 

 stitute the flower and are related to the function of gametic repro- 

 duction. Intergrades occur, a special case being that of the coty- 

 ledons, which may be related exclusively to the nutritive function, 

 or may be nutritive in the early ontogeny of the seedling, and later 

 carry on photosynthesis. Special leaf forms also occur, including 

 spines and tendrils. 



