42 



MOKPHOLOGY, OE COMPAEATIVE ANATOMY. 



Fig. 44. 



Fig. 45. 



The leaves arise from and mark the nodes of the stem ; and it 

 has been already stated that it is at the nodes, in the axils of 

 leaves, that lateral or axillary buds are as a general rule produced. 

 From this it follows that the arrangement of the leaves must be of 

 great importance, not only in reference to their own relative 

 positions, but as determining more or less completely the plans of 

 ramification of stems. It is found that the modes of arrangement 

 of leaves are in accordance with certain general laws ; and a 

 particular study of these laws has been pursued, under the name 

 of 



Phyllotaxis. Leaves exhibit two principal types of arrangement : 

 either they are solitary, 

 one only occurring at a 

 node, or two or more spring 

 from the stem at the same 

 level. When the leaves 

 stand alone, they are said 

 to be alternate or scattered 

 (fig. 44); where two stand 

 at the same level, facing 

 one another, they are called 

 opposite (fig. 45); and if 

 more than two originate 

 at one level, forming a 

 circle, the leaves are called 



ulwrled or Verticillate. Kg. 45. Diagram of the arrangement of decussate op- 



Yery rarely two leaves ap- posifce and tetrastichous leaves - 



pear to spring from, the same node, as in what are called geminate 

 leaves (Solanum). This condition is supposed to arise from irre- 

 gular displacement and partial adherence of one of the leaves to 

 the stern, or from division of one leaf into two. 



Really whorled leaves are not so common as is sometimes imagined, 

 the whorled condition being imitated in some cases, as in many Stel- 

 latae, by an excessive development of interfoliar stipules ; truly whorled 

 leaves are seen in Paris and Myriophylluin. Representatives of the two 

 principal types are found in the embryo of Monocotyledons and Dicoty- 

 ledons the former having a solitary cotyledon, the latter having two, 

 placed the one opposite to the other (fig. 3) ; but this opposite arrange- 

 ment of the cotyledons is not always associated with a like disposition of 

 the true leaves. 



Alternate leaves exhibit many modifications of arrangement. 

 Sometimes they are truly alternate ; that is, the second leaf is 

 exactly on the opposite side of the stem from the first, and the 

 third exactly over the first : a series of leaves arranged in this way 



- 44 ' Dia ,Bm of the arrangement of alternate disti- 



CHOUS 1CRV68. 



