88 



GYMNOSPERMS 



since the Bennettitales have a leaf trace passing straight from the 

 stele to the leaf. The most critical studies of the leaf trace have been 

 made by Thiessen,'"''' Dorety,"'^ and Langdon,^-'^ (figs. 78, 79, 80, 

 and 81). The leaf traces pass through the cortex, not horizontally, 

 but rising a little, so that in a thin transverse section the girdling 

 feature might be overlooked. The leaf trace often passes half-way 

 around the stem, so that it enters the leaf almost opposite the start- 

 ing place. The traces from the leaf gaps keep joining the girdling 

 trace as it passes through the cortex, making the trace larger and 



Fig. 79. — Dioon spinulosum: reconstruction of vascular system of stem of a seedling, 

 showing the girdling of the leaf traces. The two groups of five bundles each (parallel at 

 the top), are passing out to two leaves. — After Sister Helen Angela Doretv.'?^ 



larger as it nears the leaf. The trace, as it appears in a thick trans- 

 verse section, is shown in fig. 79. 



The traces consist almost entirely of scalariform tracheids. This 

 feature and their union with the xylem of the stem is shown in fig. 81. 



Such a girdling of the leaf trace is not confined to cycads, being 

 found in many angiosperms which have ''radial" leaves. 



A curious feature of the stem, when one has an opportunity to see 

 a longitudinal section of the entire trunk of a genus which has termi- 

 nal cones, is the cone dome. The first cone borne by such a plant is 

 actually terminal, but all succeeding cones, although apparently ter- 

 minal, are really lateral. The meristem is entirely used in the forma- 

 tion of the cone, and a new meristem appears at the base of the pe- 

 duncle, and from the new meristem crowns of leaves are formed 



