408 RELATION BETWEEN roSITION AND B'ORM OF GREEN LEAVES. 



RELATION BETWEEN POSITION AND FORM OF GREEN LEAVER. 



Now that the distribution of the green leaves on the surface of the stem has 

 been generally described, it is possible to discuss the relation of the phyllotaxis to 

 the length and breadth, as well as to the shape and direction, of the leaf -blades. 



If a small leafy moss-plant, or a huge densely-leaved tree be examined, it will 

 always be found that the number of orthostichies on the straight stems becomes 

 smaller as the leaf -blades become broader. If the leaf -blades are circular, like 

 those of the Judas Tree (Gerds Siliquastrum), or if they are broadly ovate or 

 cordate, being broadest at the base, like those of limes and elms, or if they are not 

 perhaps borne on very long petioles, i.e. like those of the Aspen {Poptdus tremtda), 

 then they pass down the stem in two lines, thus displaying a one-half phyllotaxis. 

 If the leaf-blades are bi'oadly elliptical, and therefore broadest about the middle, 

 and also have but short stalks, like those of beeches, alders, and hazels, then they 

 are arranged regularly in three rows on the erect branches and display a one-third 

 phyllotaxis. If the leaves are obovate, i.e. broader at the top than at the base, 

 and at the same time have only short stalks, as, for example, those of oaks, then 

 they are arranged in five Knes, according to the two-fifths phyllotaxis. If they 

 are lanceolate or oval, like those of the Almond-tree, they usually have the three- 

 eighths phyllotaxis; and finally, the narrow linear leaves on the twigs of the 

 Genista tinctoria, as well as the long narrow leaves on tlie stems of the Golden- 

 rod (Solidago), are regularly arranged in a five -thirteenths phyllotaxis. In the 

 mosses the same relations hold good ; the broad leaves of the Mnium, species 

 display the one-third; the elliptical and oval leaves of many earth-mosses (Barhula) 

 the two-fifths; and the narrow linear leaflets of polytrichums the three-eighths, 

 five-thirteenths, and more complex phyllotaxis. This connection between the 

 breadth of the leaf-blade and the number of rectilineal leaf-rows on the erect stem 

 is very noticeable even in members of the same genus, and in this respect perhaps 

 no genus is so instructive as the willow. Thei-e are willows with circular, elliptical, 

 oval, and narrow linear leaves, and in these it can be plainly seen that the number 

 of orthostichies increases in proportion as the leaves become narrower. Salix 

 herbacea with circular leaves has a one-third, Salix Caprea with elliptical foliage 

 a two-fifths, Salix pentandra with lanceolate foliage a three-eighths, and Salix 

 iTicana with linear leaves a five-thirteenths phyllotaxis. 



If we take erect branches from each of these willows, placing them all together, 

 and look down upon them from above, we see how the three, five, eight, and 

 thirteen rows of leaves radiate out from their respective axes. But it is also 

 plainly evident that in each case the neighbouring rows so adjoin one another as 

 to leave no gaps between them, so that the space round the stem may be utilized 

 to the greatest possible extent. In one case, therefore, we have three rows of very 

 broad leaves, in other cases five or eight rows of moderately broad leaves, and again 

 in another instance thirteen rows of very narrow leaves. 



