6. ARRANGEMENT OF LATERAL MEMBERS. 



18 



FIG. 5. Diagram of a stem 

 with alternate two - leaved 

 whorls : 0, 0, 0, 0, the four or- 

 thostichies : 1, 1, 2, 2, 3, 3, the 

 successive whorls. 



are arranged in alternate whorls, they form twice as many longi- 

 tudinal series on the stem as there are leaves in each whorl, pro- 

 vided, of course, that the number of 

 leaves in each whorl is the same. The 

 longitudinal series, which are indicated 

 in the diagram by radii, are called or- 

 thostichies. 



This particular arrangement of alter- 

 nate whorls of two leaves occurs very 

 frequently, and is termed the decussate 

 arrangement. The two leaves of each 

 whorl are said to be opposite. It is 

 comparatively rare for equal successive 

 whorls to be superposed ; that is, that 

 the leaves of each whorl should lie ex- 

 actly above or below those of the others, so that there are only 

 as many orthostichies as there are leaves in each whorl ; but it 

 sometimes occurs in flowers. 



Examples of decussate leaves : the Caryophyllaceae, the Labiatae, the 

 Caprifoliaceae, to which belong Syringa (Lilac), Lonicera (Honeysuckle), 

 and Sambucus (Elder); the Maple, the Horse-Chestnut, and the Ash. 

 In Rhamnus cathartica the two leaves of each whorl are usually at a 

 slightly different level. 



Alternate whorls of many leaves occur in Equisetum and Hippuris ; al- 

 ternate whorls of 3 (irrespectively of flowers) occur in the common Ju- 

 niper, in Catalpa, and occasionally in the Horse-Chestnut and the Maple. 



When successive whorls consist of unequal numbers of members, the 

 relations of alternation become highly complicated, as in the shoot of 

 Polygonatum verticilla- 



tum and in the flowers o 



of the Pomeae (Apple, 

 etc.) 



When the leaves 

 are arranged in a 

 scattered manner it 

 is easy to detect 

 that, within a cer- 

 tain region of the 

 stem, their diverg- 

 ence is constant ; 

 that is, that the dis- 

 tance between any leaf and its immediate predecessor and successor 



Fio. 6. A, Diagram of a stem bearing leaves with a 

 divergence of \ ; B, a utem bearing leaves with a diver- 

 gence of J. 



