602 



ing three leaves, forming the tristichous arrangement, are the most 

 characteristic of that class of plants. 



" The spire becomes much more complicated where it contains 

 many more elements, but the relations are found to be constant, and 

 in plants exhibiting such forms the spiral arrangement becomes 

 much more apparent, as in the Pandanus or screw-pine, the pine- 

 apple, and among the Coniferae. 



" A little consideration renders it evident how this spiral disposition 

 of the organs ensures the symmetrical distribution of the leaves, and 

 the equable formation of new^ parts deriving their assimilated nutri- 

 ment from them. A proof of this influence is afforded by the manner 

 in which the form of the stem deviates from the cylindrical in those 

 plants where the spiral arrangement of the leaves is least obvious. 

 The various organs resulting from the higher states of development of 

 the typical organ, namely, the floral envelopes, and even the carpels 

 (as in the Coniferge), are all subject to the law of spiral disposition, 

 and the importance of this view in elucidating the structure of the 

 flowers will hereafter be seen. In a large number of plants the law 

 of arrangement just described does not at first appear to hold good, 

 namely, in those which have opposite or verticillate leaves. But op- 

 posite leaves may be supposed to proceed from two spirals proceed- 

 ing up the stem simultaneously, or from the non-development or 

 shortening of the alternate internodes. The whorl or verticil^ again, 

 may be conceived to originate from the non-development of the inter- 

 nodes of a whole cycle, each spiral cycle being thus reduced to a cir- 

 cle. Examining the succeeding whorls, we do not often find the 

 leaves of the second perpendicular to the first, but with their midribs 

 over the intervals between those below. Supposing the leaves to be 

 opposite, the second pair would stand at right angles to the first {de- 

 cussate) : if the whorl contained three, it would require a greater 

 number of series before the leaves would again correspond vertically. 

 In these cases the number of cycles or whorls required to complete 

 the vertical coincidence constitutes a rectilinear series, and in oppo- 

 site leaves consist of four cycles ; where the whorls are composed of 

 three leaves, of six, and so on. If the pairs do not cross exactly at 

 right angles it will require several pairs to form a series, and these 

 pairs follow a spiral arrangement like that which is commonly found 

 in alternate leaves. 



" Some authors contend that the normal arrangement of leaves is 

 verticillate, and that the spire arises from the breaking up of this by 

 the elongation of the stem. Those, on the contrary, who contend 



