io8 THE ANATOMY OF WOODY PLANTS 



present in these ancient organisms, while others have contended 

 for a very simple type of organization in the arboreal club mosses 

 of the Paleozoic age. In view of the differences of opinion and the 

 uncertainty of the data in the case of the phloem of Paleozoic 

 Pteridophyta, it will be well to confine our attention in this 

 instance to the living representatives of the fern alliance. 



Fig. 84 shows the detailed organization of one of the fibro- 

 vascular strands of the bracken Pteris aquilina. The bundle as a 



FIG. 84. Bundle of Pteris aquilina. Explanation in the text 



whole is clearly limited by a dark uniseriate layer, the endodermis, 

 which is ordinarily interpreted as the innermost layer of the funda- 

 mental tissues in juxtaposition to the fibrovascular system. Within 

 the endodermis is situated a layer one or sometimes two cells in 

 breadth, the pericycle, which constitutes the external boundary 

 of the fibrovascular tissues, just as the endodermis marks the 

 internal limit of the fundamental system. Next to the pericycle 

 are certain extremely minute, apparently empty, cells, the first- 

 formed elements of the phloem, or, as they are technically desig- 

 nated, the protophloem. Next inward lie storage cells, nucleate 



