XXI] SPORANGIA 143 



(Vol. I, p. 162). In that Mesozoic fossil the trace is at first mesoxylic (2, Fig. 

 432): it is only after forming an internal island of parenchyma (3) that it 

 opens adaxially, so that the single protoxylem-group lies at the base of an 

 involution (4). This is the condition shown by the leaf-trace of the living 

 Osmundaceae at its first origin (compare Fig. 427, p. 136). In fact the most 

 rudimentary stages seen in the fossil have been eliminated in the fossils of 

 the later Mesozoic, and in the living representatives of the family. But the 

 subsequent subdivision of the protoxylem in the departing leaf-trace, and 

 the spreading of the whole trace into a C-shaped curve with the numerous 

 protoxylems ranged along its concave face is seen in the same way in the 

 living Osmundaceae as in Thamnopteris. The extra-marginal origin of the 

 pinna-traces is probably a consequence of the strong curvature of the parent 

 meristele. Finally the open venation of the blade accords with the proved 

 antiquity of the Family. 



The root-structure conforms to the ordinary Leptosporangiate type. As 

 a rule the stele in the root is diarch, but in Osmimda triarch roots are 

 occasionally found (Faull), and this may be held as a reminiscence of the 

 more complicated structure common in Eusporangiate Ferns. 



There are two types of hairs in the Osmundaceae, but both are composed 

 only of a simple row of cells. Woolly hairs are found on the leaf-blade, but 

 glandular hairs cover the widened leaf-bases. These have been studied by 

 Gardiner and Ito {Ann. of Bot. i, p. 41). The secretion originates from the 

 peripheral cytoplasm in a number of the cells of each hair: on access of 

 water the accumulated mucilage swells, the cells burst, and the secretion forms 

 a general protection for the leaf- base. We have learned by experience of many 

 early Ferns that such simple uniseriate hairs are a primitive type, and these 

 may also be held as such notwithstanding their specialised secretory function. 



Spore-Producing Members 



The variability in position of the sporangia, essentially marginal in 

 Osniunda and superficial in Todea, has already been noted (Fig. 420, p. 131). 

 In their development they are substantially alike whatever their position. 

 The development differs from that of most Ferns in the variety of its details 

 in different individual sporangia, even when they may be in close juxta- 

 position on the same pinnule. The sporangia fluctuate between two types, 

 as shown by the details both in Osimmda and Todea: these are illustrated 

 in Fig. 424, p. 133, which represents drawings from two actual sporangia of 

 Todea barbara. In the one type the segmentation results in a square-based 

 archesporial cell as seen in the Eusporangiate Ferns, the other shows 

 the conical type characteristic of the Leptosporangiates. The latter is 

 commoner in the Osmundaceae. The differences of individual detail start 



