44 PART I. THE MORPHOLOGY OF PLANTS. [ il. 



Branches are sometimes developed as thorns (Fig 25). Thorns are hard, 

 pointed structures ; they sometimes form the extremity of an ordinary shoot, 

 as in Rliamnus cathartica ; or they are dwarf-shoots, as in Cratagus coccinca ; 

 they may bear branches which spring from the axils of scaly leaves, as in 

 Gleditschia and the Sloe (Fig. 25). 



The morphology of the constituent members of the leafy shoot, 

 namely the stem and the leaf, will now be considered. 



11. The Stem. The stem of an annual plant or of an annual 

 shoot is succulent in texture, and is said to be herbaceous. 



A primary stem which persists for several years, though it is 

 herbaceous at first, becomes hard and woody in texture, and is 

 termed a trunk. 



The stem is commonly branched ; but it may be uiibranched, as 

 in Tree-Ferns, Cycads, many Palms and Grasses. 



The form of the stem varies very widely. It may be short &n<l 

 much thickened, as in the bulb, corm and tuber, mentioned 

 above (p. 40) and in some Cacti ; or a 

 portion of it may be much thickened 

 into a tuber, as in certain epiphytic 

 Orchids, where one or more of the basal 

 internodes form a pseudo-bulb ; and in 

 Vitis gongylodes, where any internode 

 may become tuberous. 



The form of the elongated stem is 

 commonly cylindrical or prismatic. The 

 leaf-scar, from the axil of which prismatic form is, in some cases, de- 



on^he'thorn^re 11 / 1 /' leaf "cars- tennined by the arrangement of the 

 in the axil of the upper one is leaves; thus, stems bearing decussate 

 the branch z, in that of the i eaves ( see Fiff. 11. p. 24), that is, leaves 



lower, the bud Ic. ' L ;' . 



arranged in four othosticmes, the stern 



is quadrangular. When the stem has an angular form, the edges 

 frequently grow out into a leafy expansion : such a stem is said 

 to be winged. In some cases, as in Grasses, Bamboos, Pinks, etc., 

 the stem presents a jointed appearance at the nodes ; a stem with 

 this peculiarity is termed a culm or haulm. 



When the development of the foliage-leaves of a shoot is de- 

 generate, the stem performs the functions of the leaves : it is then 

 of a green colour, and generally assumes such a form as to have 

 a relatively large surface. Thus, the whole stem and its branches 

 may become flattened, as in Opuntia (Cactacese) and in Genista 

 sagittalis (Papilionacese) : or certain branches only, termed phyllo- 



