THE ANATOMICAL STRUCTURE OF THE LEAF. 95 



third of the leaf; or, lastly, may form a narrow band down the 

 whole centre of the leaf above the midrib. Beneath the epidermis 

 a series of thickened cells is generally developed. These belong 

 to the ground tissue of the leaf, and have nothing to do morpho- 

 logically with the epidermis. These thickened cells form a continu- 

 ous, or interrupted, or scattered series called the hypoderma, and 

 in a few rare cases they are wanting altogether. 



The study of the minute anatomy of the leaf, as shown by 

 making a simple transverse section of the fully developed leaf, 

 cannot fail to be useful to all arboriculturists. After the examina- 

 tion of several hundreds of specimens, native and cultivated, I can 

 testify to the permanence of the characters, and to their value in 

 separating many closely related forms. It must be kept in mind, 

 however, that attention must be paid to the proper selection of the 

 leaves for examination. They must be full grown, and from plants 

 of some size, as the leaves of very young specimens (say two 

 or three years old) often differ from those of mature ones. The 

 differences in the structure of young and old leaves generally have 

 a certain relation. Thus a species, which has no hypoderma in 

 the mature leaf, will not have any in the young leaf ; and a leaf, 

 with a continuous zone of hypoderma, when mature, will have a 

 more or less interrupted one in the young leaves. In a few cases 

 the position of the resin canals seems a little variable in the young 

 and old leaves. This has been observed in a few cultivated forms, 

 as Abies bifolia, A. Murray, Abies firma of gardens, and in Abies 

 pichta of gardens. In some of the species, two kinds of leaves 

 are produced, those in the ordinary vegetative shoots being differ- 

 ent from those on the cone-bearing axes. This is very well marked 

 in Abies bifolia, Murray (hence the specific name), also in Pseudo- 

 tsuga magnified and nobilis, and probably also in A. Pinsapo, but I 

 have not been able to examine the cone-bearing shoot of an authen- 

 tic specimen. 



In this essay I propose to take Parlatore's species, as described 

 in De Candolle's Prodromus, and make some remarks on the syn- 

 onomy. It will be necessary to depart from his arrangement 

 somewhat, as it is of importance to separate the anatomically 

 distinct species belonging to Pseudotsuga (of which A. nobilis is the 

 type) from Abies. It is also impossible to give full details of the 

 microscopic structure of the leaves of each species, as that would 

 require more space than I have at my disposal, and would need 

 numerous drawings to render the description intelligible. It is 



