NATURAL SCIENCES OF PHILADELPHIA. 33 



ON NUMERICAL ORDER IN THE BRANCHING OF SOME CONIFERS. 



BY THOMAS MEEHAN. 



In a paper entitled " Adnation in Coniferse," read at the 

 Chicago meeting: of the American Association for the Advance- 

 ment of Science, and which was published in the "Proceedings'' 

 for 1868, I pointed out that the true leaves of Coniferae were 

 mostly adherent to the stem not merely " decurrent" as is 

 usually said of some of them ; and that the vigor of the axis or 

 stem was the measure of the adhesion. I now propose to show 

 that axial vigor also determines the law of branching in some 

 cases, and that the branching is on a numerical plan. 



In the most vigorous growths of Thuja occidentalism the com- 

 mon American arbor vita?, the leaves are almost wholly united 

 with the axis, only the delicate sharp awns are free. These are 

 arranged in pairs, one leaf opposite the other. The upper pair 

 alternates with the lower (decussate). A branch appears at the 

 eighth node ; and always at the eighth node when the vigor of 

 the branch remains the same. As the axis weakens the branches 

 appear at the sixth node. This is the general average. With 

 greater weakness the fourth node gives birth to the branch ; and 

 finally as the plant takes on its frondose flattened form, a branch 

 pushes from every alternate node. But in no case does a branch 

 push at an odd number. They are always from the second, fourth, 

 sixth, or eighth node. 



In Tfiuja gigantea, Nutt., the same law prevails, the sixth 

 and eighth being more numerous. 



In Libocedrus decurrens all appear to be on the alternate plan. 

 I have seen no instance, even in vigorous shoots, where the 

 branches push otherwise than from every second node. This is 

 also true of Chamsecyparis Laivsoniana, Pari.; and of G. obtusa, 

 Sieb., G. pisifera, and C. retusa all probably varieties of one 

 thing. 



In Biota orientalis, the branching is mostly from the fourth node, 

 occasionally from the second or sixth ; rarely one will come from 

 the third and odd number. In the curious variety B. 0. pendula, 

 Pari., where the plant has lost or never achieved the power to 



1872.] 



