302 



GENERAL BOTANY 



xylem in the ferns is a very ancient one in vascular plants and 

 gave rise to the collateral arrangement by a gradual process of 

 evolution, the discussion of which is not within the scope of an 

 elementary textbook: The cambium is entirely lacking in the 

 stem of the maidenhair and in the stems of most living ferns, 



so that no secondary phloem, xylem, 

 or wood rays are developed. Storage 

 is therefore largely limited, in all liv- 

 ing FilicaleSj to the wide cortex and 

 the large pith. 



The general form of the vascular 

 cylinder in the maidenhair, and the 

 relation of the leaf traces and leaf 

 gaps to it, are matters of considera- 

 ble interest and importance, since 

 they aid in the correct interpretation 

 of similar structures in the higher 

 plants and also give the student a 

 clearer conception of the continuity 

 of the vascular system and of the 

 gross anatomy of fern stems. 



Fig. 1 70, a, shows a portion of the 



FIG. 171. Plant of Pteris aqui- , . , , -_c -i i i 



llna (the eagle fern), showing rhlzome g rea % magnified, in which 

 rhizome and leaf the outer cortex has been removed 



The rootstock (rh) is horizontal from one side so as to expose the 

 and grows underground; upon it centra i vascular cylinder composed 



are the buds (6) and the upright j 



leafstalk (*) of xylem and phloem. By an in- 



spection of this figure it may be 



seen that a portion of the phloem and xylem ring of the cen- 

 tral cylinder passes out to form the vascular bundle of each 

 leaf petiole. This vascular bundle, or leaf trace, thus connects 

 the phloem and xylem of the main vascular ring with the leaf 

 veins. The figure shows also that the entire vascular cylinder 

 of the fern rhizome is not a completely closed cylindrical tube, 

 but is broken at intervals by leaf gaps. In fern rhizomes 

 where the leaves are large and close together the gaps are 

 more frequent, and the vascular cylinder comes to resemble a 



