MODIFICATIONS OF FORM 



201 



{decussate arrangement). This arrangement may be maintained 

 through life, as it is in the Dead-Nettie, Willow-herb (Fig. 133), Lilac, 

 Horse-Chestnut, or Sycamore. The upright shoot of the Sycamore 

 is a good example of the way in which the circular area round the axis 

 is put to the best use by leaves arranged on a decussate plan. Each 

 successive pair fits into the gap between those of the preceding pair. 

 But if the internodes were short, as they are in the young state, the 

 higher would overshadow the next pair but one of lower leaves. 

 This difficulty is met by the lower pair having longer petioles, so that 



Fig. 134. 



Young leafy shoot of Sycamore seen from above : showing how with very little 

 overlapping the leaf-blades form a mosaic. The spaces unoccupied centrally will 

 be filled as the younger leaves expand. 



their blades are carried out beyond those of the leaves immediately 

 above them, forming a compact " leaf-mosaic " (Fig. 134). 



The decussate is the simplest of the cyclic or whorled arrangements, 

 where two or more leaves are seated at the same level. But in 

 other cases the number of the leaves at the same level may be 

 not two only, but three, four, or more. As in the decussate plan the 

 leaves of each succeeding whorl alternate as a rule with those of the 

 preceding, so that they occupy the spaces between them, an arrange- 

 ment that is very convenient in the packing of the crowded parts into 

 small compass in the bud. A transition to higher numbers in the cycle 

 may be seen in the individual plant. Thus in Fuchsia, which has 

 usually decussate leaves, a very strong shoot may bear alternating 

 whorls of three. In Lysimachia vulgaris, and in the Privet, a like 



