PTERIDOPHYTES 



301 



composed of storage parenchyma, in which the reserve food is 

 stored for the spring growth. The vascular system is composed of 

 a phloem and xylem cylinder which runs lengthwise of the stem. 

 From this vascular cylinder strands called leaf traces pass out 

 through the cortex into the leaves. Where a leaf trace passes 

 off from the vascular cylinder a break called a leaf gap occurs 

 (Fig. 169). The roots also connect with the central vascular cylin.- 

 der, so that a continuous vascular system exists in the maidenhair 



-^Skeletal tissue 

 Storage tissue \ 

 Phloem^ \ N 

 Xylem 



tfortex 



idermis 



Pith 



-Epidermis 

 Cortex 



"Leaf trace 1- 



~Lcaf trace / 

 ^Leaf petiole^ / 

 ^^Leaf gap 



FIG. 170. Gross anatomy of a portion of the rhizome of Adiantum 



a shows the relation of the vascular system to the other tissues and to the leaves 

 (note the breaks or leaf gaps in the vascular cylinder, caused by the exit of the leaf- 

 trace bundle ; the cortex has been removed from one side to expose the wood cylinder, 

 leaf gaps, and leaf traces in surface view) ; b, transverse section of the rhizome cut in 

 the region of a leaf gap and a leaf trace 



and other ferns as in the higher plants. The arrangement of the 

 tissues in a transverse and a longitudinal section of the rhizome is 

 shown in Fig. 170. Fig. 170, , shows the concentric arrange- 

 ment of phloem and xylem which is characteristic of fern stems. 

 In this arrangement the phloem completely incloses the xylem, 

 flanking it on the outside next to the cortex and on the inside 

 next to the pith. In the seed plants, including the alder and Salvia 

 (Fig. 56), it will be remembered that the arrangement was col- 

 lateral-, that is, with the xylem on the inside of the vascular 

 cylinder next to the pith, and with the phloem on the outside 

 next to the cortex. The concentric arrangement of phloem and 



