SPERMATOPHYTA 



353 



c J^ II 



h i — 



e — «t — 



outer one often disappearing entirely. Thus the seed coat consists essen- 

 tiall}^ of the middle stony layer. The embryo goes into a condition of 

 dormancy in which it usually remains for many months, although the 

 seeds of many conifers will germinate without undergoing a resting period. 

 As in the cycads and Ginkgo, food is stored in the vegetative tissue of the 

 female gametophyte, generally designated as endosperm. In germination 

 the entire embryo emerges from the 

 seed coat, which is carried out of the 

 ground on the tips of the cotyledons. 



Summary. The Coniferales were 

 probably derived from the Cordai- 

 tales during the Paleozoic and repre- 

 sent a parallel line of evolution to the 

 Ginkgoales, which seem to have had 

 a similar origin. These three orders, 

 comprising the coniferophj^tes, con- 

 stitute a line of descent as old as the 

 cycadophytes. The Coniferales have 

 retained fewer primitive reproductive 

 features than the Ginkgoales, their 

 chief advance being in ovule struc- 

 ture and the loss of swimming sperms. 

 In both orders the stem, freely 

 branching, is an endarch siphonostele 

 with almost no mesarch structure 



left. Moreover, the pith is small and the vascular cylinder thick. The 

 leaves of conifers are characteristically small and without dichotomous 

 venation. Although the microsporophylls are grouped to form a simple 

 strobilus, the ovulate strobilus, except where greatly reduced, is com- 

 pound, a feature also of the Cordaitales. 



The six families of conifers represent various degrees of progress from 

 a common ancestry. Their advance has been in different directions, as 

 follows:^ (1) The arrangement of leaves and sporophylls is spiral in all the 

 families except the Cupressaceae, where it is cyclic. (2) All families have 

 distinct ovulate cones except the Podocarpaceae and Taxaceae. (3) The 

 bract and scale in the ovulate cone are separate in the Abietaceae but 

 united in the other families. (4) Winged pollen grains are present only in 

 the Abietaceae and Podocarpaceae. (5) A considerable amount of 

 vegetative tissue is present in the male gametophyte of the Araucariaceae 

 and Podocarpaceae. Two ephemeral prothallial cells are formed in the 

 Abietaceae, none in the three other families. (6) A ventral canal cell is 

 formed in the Abietaceae, only a ventral canal nucleus in the other fami- 



1 In each case the condition to be regarded as primitive is stated first. 



A B 



Fig. 301. Embryo (A) and longitudinal 

 section of the seed (B) of Pinus edulis, 

 X4; 0, outer seed coat; i, inner seed 

 coat; c, cotyledons; s, stem tip; h, 

 hypocotyl; r, root tip; e, endosperm. 



