JOURNAL 



OF THE 



ARNOLD ARBORETUM 



Volume V OCTOBER, 1924 Number 4 



NOTES ON THE GENUS PINUS 



George Russell Shaw 



THE OBLIQUE CONE 



To the characters that authors employ for circumscribing Pinus, should 

 be added the tendency of the genus to produce oblique cones. This form 

 of cone is usually described as asymmetrical, but it is not absolutely 

 without symmetry, for it may be divided into two like parts by a plane 

 passing through the axes of both the cone and its branch, while the cones 

 of Picea and other allied genera are symmetrical with reference to any 

 plane that includes the cone-axis. 



In Pinus the cones grow from the nodes only of the branch and, when 

 the branch let bears more than one cone, they are disposed in verticillate 

 groups. The reflexed oblique cone fits perfectly into the verticillate 

 arrangement, the thickness and the size of the scales varying with the 

 difference of exposure. This peculiar elaboration, in its perfection, appears 

 among the serotinous Pines whose cones persist on the tree and retain 

 their seeds intact for many years. 



In its least development the oblique form may be seen in the curved 

 cones, or in the eccentric insertion of the cone-peduncle, of many species of 

 both sections. It is the result of a lesser or greater inequality in the growth 

 of the anterior and posterior tissues. In some degree it is present in all, 

 or nearly all of the Pines and various degrees of obliquity are associated 

 in the same species. It is sufficiently prevalent to be accepted as a 

 characteristic peculiarity of the genus. 



SECTIONAL CHARACTERS 



The characters determining the two sections of Pinus are many but 

 they are not all absolutely invariable. 



Haploxylon 



Fibro-vascular bundle of the leaves single. 



Sheath of the leaf-fascicle deciduous. 



Bracts subtending the leaf-fascicles not decurrent on the branchlet. 



Staminate flowers not sufficiently developed in the winter-bud to be 



apparent. 

 Connective of the pollen-sacs relatively small. 



