83 PBAGTIGAL BOTANY. 



lines on the axis (stem or branch). Leaves so arranged 

 are said to be decussate. In like manner, whorls of 3 or 

 4 leaves will occupy 6 or 8 lines of this sort. 



157. But most plants have alternate leames ; and the 

 arrangement of these will be best understood by reference 

 to certain ingenious distinctions, drawn by botanists. 



In the Oak, five leaves are spirally arranged around the 

 axis, so that the sixth, succeeding the fifth, is placed in the 

 same vertical or longitudinal line above the first. Suppose 

 we examine a branch of some length : the seventh leaf will 

 be placed above the second, and the eighth above the third. 

 The same arrangement is seen in Prunus, Crataegus, Khns, 

 Salix, Populus, etc. 



138. If we wind a thread around a branch of Hazel or 

 Elm, so as to touch the insertion of the several leaves, 

 the turns or revolutions of this thread will make a regular 

 spiral. Taking one of the leaves as a starting-point, and 

 counting those above it, we find that the third leaf is 

 placed directly above the first, the fourth above the 

 second, etc., and that all these leaves are arranged in two 

 longitudinal lines or rows, separated by half a circum- 

 ference of the branch. Such leaves are said to be disti- 

 chous or two-ranked. This mode of arrangement occurs in 

 all grasses and many other endogens, but also in certain 

 exogens, as Hazel, Elm (as was mentioned above), Linden, 

 some Leguminosse, etc. 



159. If, in the same way, we wind a thread around a 

 branch of Birch, the fourth leaf is found to be directly 

 above the first, the fifth above the second, etc. They are 

 all on three equidistant vertical lines, and separated by 

 one third of the circumference of the axis, ' In this case 

 we say that three leaves are in one cycle, and that the 



