5U 



BOTANY OF THE LIVING PLANT 



after the manner of Ferns. In the larger fossils, however, cambial 

 increase was introduced, providing a massive tract of secondary 

 wood, to serve the upright and tree-like trunks of Lepidodendron and 

 Sigillaria. A vestige of such structure still persists in the abbreviated 

 and succulent stock of Isoetes. 



The greatest interest lies not in the structure of the Ligulatae, but 

 in their sporangia, and the germination of the spores : for these give 

 lines of comparison with the Seed-Plants on the one hand, and with 

 homosporous types on the other. The strobilus or cone that bears 

 them is distal on a vegetative branch, and even in the flattened 



Fig. 405. 



Part of the shoot of Selaginella Martensii, showing its " espalier " form and minute 

 unequal leaves. It is seen from above, and the forking rhizophores are directed 

 downwards. (Nat. size.) (After Goebel • from Strasburger.) 



species of Selaginella it has the radial form, all the sporophylls being of 

 equal size (Fig. 406, A). A longitudinal section shows that a short- 

 stalked sporangium is borne in the axil of each. These sporangia 

 are of two sorts, which are associated in the same strobilus but dis- 

 posed in various ways in different species. In S. inaequalifolia those 

 on the right side of the section shown in Fig. 406, B, are all mega- 

 sporangia, with four large spores in each ; those on the left-hand side 



