32 



pronged spine and on one and the same shoot intermediate 

 forms between the typical spine and normal foliage leaf can 

 often be found, see Fig. 3, Plate VI. 



That these spines are really leaves is indicated by the fact 

 that buds and branches arise in their axils just as they do in 

 the axils of true leaves ; moreover the base of the spines is 

 furnished with minute stipules j list as are those of the normal 

 leaves. 



31. The way in which the leaves 



are arranged upon the stem is called phyllotaxy, or phyttotaxis. 

 Some of the arrangements commonly found, such as opposite 

 leaves, alternate leaves, and so on, have already been noted 

 above, but it is usual to distinguish some additional types of 

 the alternate arrangement by fractions, such as ^, f, f and 

 so on. To determine the phyllotaxy of a plant one leaf is 

 selected on the stem as a starting point which we may 

 call (a) and then, passing up the stem, we note the first 

 leaf which is situated exactly vertically above it which may 

 be called (b). A spiral line is then traced from the start- 

 ing point (a), passing the shortest way around the stem 

 through the insertions of all the leaves situated between (a) 

 and (b) in consecutive order and ending at (b). If this line is 

 found to have passed twice around the stem and to have 

 passed through 5 leaves, omitting (a) but including (6), the 

 phyllotaxy is said to be -f ; if the line has passed three times 

 around the stem through 8 leaves it is f and so on. When 

 alternate leaves are distichous, therefore, the phyllotaxy is ^ 

 and when they are tristichous, i.e. in three vertical ranks, it is |-. 



The phyllotaxy occasionally varies on one and the same 

 plant ; thus on erect branches of Coriaria nepalensis the leaves 

 are decussate in 4 vertical ranks, whereas on horizontal branches, 

 by a twisting of the stem, they become distichous and the 

 leaves are also twisted so that their blades are practically 

 horizontal, i.e. with the leaf-surface perpendicular to the 

 direction of the rays of light coming from above, see Plate 

 XIII. 



We have noted already how important it is for plants to 

 secure a suitable degree of illumination for their green leaf- 

 surface, and this necessity is to a great extent responsible for 

 the different forms of leaves and for the way in which they 

 are arranged upon the stem. It must, however, always be 

 remembered that all plants have not identical requirements, 

 that, for instance, the most suitable degree of illumination for 

 one plant may be injurious to another, and secondly that the 



