58 MORPHOLOGY OF SPERMATOPHYTES 



iar double leaf of Sciadopitys and phylloclad of Phyllodadus', 

 the deciduous habit of La-rix; the concrescent leaves of many 

 Cupresseae, are all derived characters, from forms with spread- 

 ing and persistent needle leaves on all the shoots. 



The histological features of the adult stem are quite uni- 

 form, and in some particulars peculiar to the group. As was 

 stated under the Cycads, however, the apex of the stem 

 is variable in character. According to De Bary 27 (page 14), 

 the dermatogen, periblem, and plerome either merge in a com- 

 mon group of initials, as in the Abieteae, or these regions are 

 distinct at the apex, each with its own initials, as in certain 

 species of Araucaria, Dammara, and Cunninghamia. One of 

 the most striking features of the stem, in contrast with that of 

 Cycads, is the persistence of the primary cambium cylinder, 

 resulting in annual increase of diameter through the accumula- 

 tion of secondary xylem, a character which constitutes the chief 

 resemblance of Conifers to Dicotyledons. The primary bundles 

 contain true tracheary tissue, but w r ith the appearance of the 

 secondary xylem the resemblance to Dicotyledons vanishes, as it 

 consists exclusively of radially arranged tracheids with bordered 

 pits, and is penetrated by narrow and vertically short pith rays. 

 In Pinus, Picea, Larix, Pseudotsuga, and probably other gen- 

 era, the pith rays are of two general types, either very narrow, 

 often but one cell broad, or lenticular in tangential section, 

 with a resin duct in the middle. In vertical extent the number 

 of cells is quite variable, probably three to sixteen representing 

 the average range of numbers. De Bary 27 (page 490), reports 

 one to tw r elve as the number of cells in the vertical plane of the 

 pith rays of some Abieteae ; while in Cedrus and in certain fossil 

 forms the number may rise as high as fifty. The bordered pits 

 are usually developed only upon the tracheid walls toward the 

 pith rays, and are discovered, therefore, by radial longitudinal 

 sections (Fig. 43). The cells of the pith rays also have large 

 closed pits upon the walls adjacent to th,e tracheids. All of the 

 bundles are common, a single strand connecting with each leaf, 

 none of them being cauline as in Cycads. Companion cells are 

 also lacking, as in Pteridophytes. The open collateral bundles 

 have retained no trace of a concentric origin, as in the Cycads, 

 unless the peculiar bundles found by "Worsdell 48 in the cotyle- 

 dons and fleshy seed integument of C eplialotaxus may be re- 



