PHYLLOTAXIS OK LEAF-AEEANGEMENT. 107 



arrangement of whose leaves is alternate. Thus, in fig. 211, if the 

 space between 1 and. 2 were obliterated, or the intemode, m, not 

 developed, the leaves would be opposite. In fig. 214, if the spaces 

 between each of the leaves were obliterated, there would be a verticil 

 of five leaves. In many plants there is a law of arrestment of 

 development, by which opposite and verticUlate leaves are naturally 

 produced : but in such cases the alternation is still seen in the 

 arrangement of the different clmters of leaves. 



In some cases the effect of interruption of growth, in causing 

 alternate leaves to become opposite and verticUlate, can be distinctly 

 shown, as for instance in Rhododendron ponticum. In other cases, 

 parts which are usually opposite or verticiUate become alternate by 

 the vigorous development of the axis : and on different parts of the 

 same stem, as in Lysimachia vulgaris, there may be seen alternate, 

 opposite, and verticiUate leaves. When the interruption to develop- 

 ment takes place at the end of a branch the leaves become fasciculate 

 {fasciculus, a bundle) or clustered, as in the Larch. A remarkable 

 instance of the shortening of internodes and the clustering of leaves 

 occurred in the Palm-house of the Botanic Garden of Edinburgh, in 

 the case of a Bamboo, which was exposed for many months to a low 

 temperature, during the time that the roof of the house was being 

 renewed. The plant had been growing rapidly, with its internodes 

 of the usual length, but it was suddenly arrested near the summit, 

 the internodes became graduaUy shortened, till the nodes were close 

 to each other, and the leaves came oflf in bunches. AU modifications 

 of leaves follow the same laws of arrangement as true leaves — a fact 

 which is of importance in a morphological point of view. 



In Dicotyledonous plants, the first leaves produced, or the 

 cotyledons, are opposite. This arrangement often continues during 

 the life of the plant, but at other times it changes. Some tribes of 

 plants are distinguished by their opposite or verticUlate, others by 

 their alternate, leaves. Labiate plants have decussate leaves, while 

 Boraginacese have alternate leaves, and Tiliacese ,usually have distichous 

 leaves ; Cinchonacese have opposite leaves ; Galiacese, verticUlate. 

 Such arrangements as f, |, ^\, and -f^, are common in Dicotyledons. 

 The first of these, called quincunx (quincunx, an arrangement of five), 

 is met with in the Apple, Pear, and Cherry (fig. 214) ; the second, in 

 the Bay, Holly, Plantago media ; the third, in the cones of Pinus 

 (Abies) alba (fig. 217) ; and the fourth, in those of the Pinus (Abies) 

 Picea. In Monocotyledonous plants there is only one seed-leaf or 

 cotyledon produced, and hence the arrangement is at first alternate ; 

 and it generally continues so more or less. Such arrangements as 

 J, J (fig. 215), and |, are common in Monocotyledons, as in Grasses, 

 Sedges, and Lilies. In Acotyledons the leaves assume all kinds of 

 arrangement, being opposite, alternate, and verticiUate. It has been 



