28 PART I. MORPHOLOGY. [ 12 



and in Vitis gongylodes, where any internode may become tu- 

 berous. 



The form of the elongated stem is commonly cylindrical or 

 prismatic. The prismatic form is, in some cases, determined by 

 the arrangement of the leaves ; thus, stems bearing decussate leaves 

 (see Fig. 4, p. 12), that is, leaves arranged in four orthostichies, 

 are 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 (tumid) 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 phyllodades, are flat- 

 tened and leaf-like as in Ruscus 

 (Liliacese), Phyllanthus (Euphorbi- 

 acese), Miihlenbeckia (Polygonacese), 

 Carmichaelia (Papilionacese), Phyl- 

 locladus (Coniferse), and are either 

 FIG. i7.-Ph y iiociade of Ruscus isobiliterally or dorsiventrally sym- 



Hvpo^um (nearly nat. size) : . met rical. The phylbclades fre- 

 stem; b leaf, m the axil of which the r J 



phyiiociade p is developed ; d leaf of quently bear flowers, but not always 



the phyiiociade bearinR flowers bl in i n the Same position. Thus, in 

 its axil. 



Ruscus androgynus the flowers are 



borne on the margin of the phyiiociade ; in Ruscus aculeatus and 

 R. Hypoglossum, they are borne on the upper surface of the phyi- 

 iociade ; and in R. Hypophyllum, on the under surface. 



Leaf-like branches are also formed in Asparagus ; they are not 

 flattened, but are small and acicular ; something of the same kind 

 also occurs in Equisetum. 



12. The Leaf. All leaves, except the primary leaves or 

 cotyledons, are developed exogenously as lateral outgrowths upon 

 the growing-point of a stem. 



In most plants the leaf undergoes differentiation or segmentation 

 along its longitudinal axis or phyllopodium. In the most com- 



