294 



GYMNOSPERMS 



copod level the amount of sporogenous tissue is not only greatly re- 

 stricted, but it appears much later in the individual life-history than 

 in Riccia. In the conifers the comparative amount of sporogenous 

 tissue is still more restricted, and it appears still later in the indi- 

 vidual life-history. 



Whatever may have been the origin of sporogenous tissue in the 

 liverworts, or, even earlier, in the algae, the spore-bearing structures, 

 from the lycopods up, are modifications of vegetative structures. In 



Lycopodium lucidulum the sporo- 

 phyllsarelike the vegetative leaves, 

 and practically all the leaves are 

 sporophylls; in L. inundatum the 

 spore-bearing leaves are somewhat 

 modified, are confined to the up- 

 per part of the shoot, and are 

 grouped into a loose strobilus. In 

 lycopods of the L. clavatum type 

 the sporophylls are quite different 

 from the vegetative leaves, and are 

 grouped into a compact cone. 



The cones of conifers, both ovu- 

 late and staminate, frequently pro- 

 liferate, changing from the repro- 

 ductive to the vegetative phase (lig. 

 299). The proUferating branch 

 may again bear cones. If an ovu- 

 late cone, like that of Pinus, is a modified shoot, the modification 

 has been extreme (fig. 298). At the top of the figure, the young si)ur 

 (5) looks much like the debatable structure (s) in fig. 297. Histologi- 

 cally, the young spur shoot and the debatable ovuliferous structure 

 look alike (fig. 300). The figure, although drawn from a young ovu- 

 late cone, might pass for a young vegetative spur in the axil of its 

 bract; but, later, the young spur produces scale leaves, and finally 

 a pair of needle leaves, while, in the young cone, the axillary struc- 

 ture produces no scale leaves, but bears two ovules on the face di- 

 rected toward the axis of the cone. 



The structure has been called an ovuliferous scale, a flattened 



Fig. 299. — Proliferating cones: A, 

 Larix heterophylla; B, Crypiotneria ja- 

 pouica; natural size. 



