Lecture XXL 173 



nected with the inner surface of the tube and take their origin 

 from the outside of the conducting tract. The conducting tract 

 resembles one taken from Aspidium. It consists of a strap 

 of wood oval in cross section encased in a sleeve of bast The 

 greater part of the strap of wood is made up of scalariform 

 tracheids, one or both of its edges are occupied by a thread of 

 spiral and annular tracheids. These form the protoxylem, and 

 if we examine a conducting tract near to the growing end of a 

 branch we will find that the protoxylem threads are differ- 

 entiated before the rest of the wood which develops subsequently 

 towards the axis of the tract. A tract developing in this 

 manner, the protoxylem being at the outside, is called exarch. 

 These exarch conducting tracts are not uncommon among the 

 Ferns but are seldom found in the stem among the Seed Plants. 

 The bast contains sieve-tubes like those of Aspidium. The 

 elongated cells suspending the conducting tract in the tube formed 

 by the cortex correspond to the endodermis of Aspidium. 



The structure of the rhizophores and roots resemble in a general 

 way that of the stem, except that the cortex fits closely on the 

 conducting tract, and there is little of no trace of the cavity sur- 

 rounding the tract. Further, there is but one thread of protoxylem 

 in the wood and it is laterally placed. In addition to these 

 differences the root carries a root-cap of loose cells covering its 

 growing apex. The growing region of the rhizophore is naked 

 while that of the stem is covered over by the young leaves. 



The leaves of the cones have a structure similar to those on the 

 prostrate stems, but in the angle formed between them and the 

 axis of the cone there is a single globular sporangium, attached to 

 the angle by a short stalk. When the sporangia are mature it is 

 seen that they are of two kinds : the smaller are smooth and egg- 

 shaped ; they are called microsporangia, while the larger, which 

 are trilobed, are called megasporangia. Each kind breaks open 

 along a line of weakness running on the sporangium at right angles 

 to the direction of the mid-rib of the subtending leaf, or sporophyll, 

 splitting the sporangium in half. From the microsporangia large 

 numbers of microspores are shed, while from the megasporangia 

 four megaspores come. Both the microspores and megaspores 

 show from their shape that they have been formed in tetrads in a 

 common mother-cell, being tetrahedral in form and having three 

 flat converging sides and one rounded or spherical side. As a 

 matter of fact the early stages in the development of the two kinds 

 of sporangia are identical. Each originates as a stalked globular 

 mass of sporogenous cells covered over with two layers of cells 



