92 I'HYLLOTAXIS OR LEAF-ARRANGEMENT. 



cases the alternation is still seen in the arrangement of the different 

 clusters of leaves. 



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

 alternate leaves to become opposite and verticillate, can be distinctly 

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

 parts which are usually opposite or verticillate, 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 verticillate leaves. When the interruption to development 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 gradually shortened, till the nodes were close to each other, 

 and the leaves came off in bunches. All modifications of leaves follow 

 the same laws of arrangement as true leaves a fact which is of im- 

 portance in a morphological point of view. 



176. In Exogenous 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 verticillate, others by 

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

 Boraginacese have alternate leaves, and Tiliaceaj have distichous leaves 

 in general ; Cinchonacea? have opposite leaves ; Galiacea3, verticillate. 

 Such arrangements as f, f , / 4 , and ,f T , are common in Exogens. The 

 first of these, called quincunx (quincunx, an arrangement of five), is met 

 with in the Apple, Pear, and Cherry (fig. 198); the second, in the Bay, 

 Holly, Plantago media; the third, in the cones of Pinus alba (fig. 201); 

 and the fourth, in those of the Pinus Picea. In Endogenous 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 \ \ (fig. 199), and f, are common in 

 Endogens, as in Grasses, Sedges, and Lilies. In Acrogens, the leaves 

 assume all kinds of arrangement, being opposite, alternate, and ver- 

 ticillate. It has been found in general that, while the number 5 

 occurs in the phyllotaxis of Exogens, 3 is common in that of Endogens. 



_ 177. Although there is thus, in the great divisions of the vegetable 

 kingdom, a, tendency to certain definite numerical arrangements, yet 

 therearemany exceptions. Inspeaking of Palms, which are endogenous 

 plants, Martius states that the leaves of different species exhibit the 

 following spirals , j, f, g, |, ^, ^ }i . ^ tne spe cies of the genus 



