. The anatomy of woody plants. Botany -- Anatomy. 2l6 THE ANATOMY OF WOODY PLANTS in the present connection to begin the discussion of the sporangium with the consideration of the situation presented by the lowest living gymnosperms, the Cycadales. Fig. 160 shows the organiza- tion of a microsporangium in Zamia muricata. The structure in question is covered on the outside by an envelope of thick-walled cells which in the condition of maturity determine its dehiscence. The mechanical structure is known as the annulus and is of great importance in bringing about the distribution of the spores, p
. The anatomy of woody plants. Botany -- Anatomy. 2l6 THE ANATOMY OF WOODY PLANTS in the present connection to begin the discussion of the sporangium with the consideration of the situation presented by the lowest living gymnosperms, the Cycadales. Fig. 160 shows the organiza- tion of a microsporangium in Zamia muricata. The structure in question is covered on the outside by an envelope of thick-walled cells which in the condition of maturity determine its dehiscence. The mechanical structure is known as the annulus and is of great importance in bringing about the distribution of the spores, particu- larly in the lower Vasculares, in which it takes the place of the elaters found in many of the liver- worts. The annulus is plainly an epidermal structure, both because it is actually the external layer of the sporangium and because its con- tinuity is interrupted by the pres- ence of stomata. These can be seen in the figure in profile view. The situation in regard to the annulus in the Pteridophyta may be briefly summarized. In lower forms the thick-walled epidermal cells which serve as the mechanism for the opening of the sporangium are massive in their development, while in the higher forms of the vascular cryptogams the amount of mechanical tissue tends to become more and more restricted. Fig. 1610 shows the structure of the sporangium and its annulus in Selaginella. The mechanical layer in this case is extensive and is almost coextensive with the surface of the spore sac. In contrast to the conditions shown in Selaginella are those presented by many of the ferns. In Fig. i6ib is reproduced the organization of the sporangium of Polypodium vulgare as an illustration of the higher type of annulus in the Pteridophyta. The opening mechanism here constitutes an incom- plete vertical ring and in consequence literally merits the name of annulus. The further consideration of the types of annulus. FIG. 160.—Sporangium of Zamia Please note that these images are e
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