XLni] FAMILY CHAEACTBES 143 



the pits on th.e medullary-ray cells, whatever relative value we 

 may assign to this character, are in many cases of considerable 

 assistance even though we are not prepared to follow Gothan to 

 the full extent of his trust in the taxonomic importance of medul- 

 lary-ray pitting. 



Dr Groom^ has described the northern evergreen Conifeis as 

 architectural xerophytes having xeromorphic leaves with a xero- 

 phytic structure. He discusses in his remarks on the Ecology of 

 Conifers the correlation of the characteristic wood-structure and 

 the xeromorphic leaves, the cause of the survival of the Coniferae 

 in competition with Dicotyledons, and other questions of interest 

 to the student of the evolution and past history of the group. 



Short summary of the . characteristics of recent Conifers'^'. 



ABAUCARINEAE. There is a close agreement in the structure of the 

 wood in the two members of this family. Attention has already been called to 

 the normal type of pitting of the tracheids and to certain exceptional forms. 

 The occasional tendency towards a scalariform type of pitting (fig. 691, D) is / 

 an interesting point. Pits are fairly abundant on the tangential walls of the^ 

 xylem-elements. In the wood of Agathis robusta^ vertical rows of parenchyma 

 are said to be fairly abundant. Annual rings not infrequently absent or 

 feebly developed. Medullary rays one-cell broad, rarely double, usually 

 7 — 15 cells deep ; but in Araucaria the depth may reach 26 cells : the ray cells 

 occasionally present a distended appearance in tangential sections of the wood 

 (cf. Ginkgo biloba). The persistence of the leaf-traces in the old wood of 

 Araucaria is a striking feature* considered by Lignier to possess diagnostic 

 importance: Jefirey^ states that traces are much less persistent in some 

 young stems. In Aravcaria each trace arises as a single strand, but in Agathis 

 it leaves the perimeduUary region as a double bundled 



Agathis (fig. 695). Leaves sessile or slightly petiolate, opposite, sub- 

 opposite oi;, on the main axis, spiral ; ovate, broadly lanceolate {A. loranthi- 

 folia, 13'x 5 cm. ; A. macrophylla 17x5 cm.). Resin-canals between the veins ; 

 transfusion-tracheids fairly abundant, but less prominent than in Araucaria. 

 The almost spherical megastrobOi (fig. 696) are very characteristic; they 



1 Groom (10). 



^ In addition to the text-books on Conifers by Beissner, Veitch, and the account 

 in Die Naturlichen Pflanzenfamilien the student should consult the Report of the 

 Conifer Conference, Journal of the Royal Horticultural Society, Vol.xiv. 1892. 



' NoeUe (10). 



* Thiselton-Dyer (01^); Seward and Ford (06) B. 



s Jeffrey (12) p. 565. ° Thomson (13) p. 15. 



