140 CONIFERALES (RECENT) [CH.- 



occur in Ahies^ and several members of other families. Chrysler^ 

 states that Cedrus differs from Abies in having ray tracheids mixed 

 with marginal parenchyma, and at the limit of an annual ring the 

 marginal tracheids may be replaced by shadowy cells or ghosts 

 of cells. The tracheids may have smooth walls as in Cedrus, 

 Tsuga, Larix, Pinus Strobus, or, as in other species of Pinus 

 ('Hard Pines'), their walls are characterised by irregular ingrowths 

 or pegs. Wettstein^ states that in Picea omorica horizontal 

 tracheids though common in the main stem do not occur in the 

 rays of branches. De Bary* recognised tracheids in the rays of 

 Sciadopitys ; they have been recorded also in Juniperus, Cupressus, 

 Thuja, Sequoia^ and, as the result of wounding, in Cunninghamia^. 



The occurrence of idioblasts in the form of irregular thick- 

 walled elements is characteristic of the pith and cortex of Arau- 

 caria, but similar cells are found in the pith of Torreya nucifera, 

 Podocarpus neriifolia, Dacrydium cupressinurnP and Cryptomeria. 

 In some cases, e.g., Abies magnificat, Picea omorica, horizontal 

 rows of thick-walled cells form diaphragms in the pith. The 

 structure of the secondary phloem has received relatively little 

 attention and owing to its comparatively rare preservation in 

 fossils it is less important to the palaeobotanist. In the Cupres- 

 sineae the regular alternation of tangential rows of hard and soft 

 bast is a characteristic feature, while in the Abietineae the phloem 

 consists of sieve-tubes and parenchyma with a few scattered 

 stone-cells. The absence of albuminous cells in the medullary 

 rays of the phloem region in the Araucarineae is noteworthy and 

 Thomson^ states that he found none in Podocarqjus. 



The structuie of the stomata in the leaves of Conifers is fairly 

 uniform: their distribution should be noted though this in itself 

 is not of much value as a distinguishing feature. They may be 

 confined to regular bands {Abies, etc.) or grooves (Torreya, etc., 

 fig. 694, B), or irregularly distributed. The position and number 

 of resin-ducts is often a useful guide : to quote one example only, 

 in the leaves of Agathis and Araucaria (fig. 694, C) the ducts 



1 Thompson (12). 2 Chrysler (15). 



2 Wettstein (90) p. 511. 



* De Bary (84) A. p. 490. See also Tassi (05) quoted by Vierhapper (10). 



5 Gordon, M. (12); Jones (13^). " Jeffrey (08). 



' Kubart (11^) s Jeffrey (05). 9 Thomson (13) p. 31. 



