536 FILICALES 



To form an estimate, the number of spores produced in a single 

 sporangium may be actually counted, or an estimate of the potential 

 number may be based upon 'the number of spore-mother-cells as shown 



in sections. By the latter method, as applied 

 to Osmunda regalis, since the number of spore- 

 mother-cells in a median section is 30 to 32 

 (Fig. 297), and the sporogenous mass is 

 approximately spherical with the diameter of 

 each cell about Jth that of the whole sphere, 

 the total number would approximate to 128, 

 though probably somewhat below that number. 

 Actual countings of spores showed figures 

 about midway between 2.56 and 512, distinctly 

 below the estimate of Russow. In Todea 

 barbara, while some sporangia approximate to 

 those of Osmunda, others fall far short, and 

 approximate to 256 ; but in T. superba and 

 hymenophylloides, which are "filmy" in habit, 

 the out ? ut is sti11 lower > approximating in the 

 last species to 128. The bearing of these 



facts will be discussed below; the results are such as coincide readily 

 with the relatively robust structure and variable details of the sporangial 

 development. 1 



ANATOMY. 



The vascular structure of the axis in the Osmundaceae has long been 

 an object of interest, on the ground of its distinctness from that of other 

 Ferns, and its apparent analogy with that commonly seen in Dicotyledons. 

 But in the form of the leaf-trace, and the way in which it breaks up as 

 it passes into the leaf, the Osmundaceae present features which are more 

 readily matched among known Filicineous types. Starting from the peri- 

 phery of the leaf, the numerous forked veins fuse, on passing downwards 

 from pinnule and pinna to leaf-stalk, into a single half-cylindrical strand 

 with its concave channel directed adaxially. This strand consists of a 

 continuous band of metaxylem, with numerous protoxylem-groups at its 

 concave limit, which alternate in position with groups of % mucilage-sacs. 

 Peripherally is a mantle of phloem, thicker on the adaxiaL side, and 

 surrounding the whole is a continuous endodermis. As it passes down to 

 the base of the petiole the strand contracts, and the protoxylems unite 

 into a single one in a median adaxial position, while in transverse section 

 it assumes a deep U-shaped outline (Fig. 298 A, B). It is in this neighbour- 

 hood that the vascular supply to the roots is given off laterally from the 

 strand of the leaf-trace (Fig. 298 A HI.). The strand thus contracted, after 

 entering the axis, takes its place in a ring of similar traces surrounding a 

 1 For a more full statement see Stiidies, iv. , pp. 38-42. 



