PTERIDOPHYTES 139 



111, 119), as it is in an unfavorable position in reference to 

 the dorsiventral stem, which puts out a series of more favor- 

 ably placed secondary roots into the soil (Fig. 118). The 

 mature leafy sporophyte, therefore, has neither foot nor 

 primary root, the product of two of the quadrants of the 

 embryo having disappeared. 



The secondary roots put out by the stem are small, and 

 do not organize an extensive system, but they are interest- 

 ing as representing the first appearance of true roots, which 

 therefore come in with the vascular system. In the lower 

 groups the root function of absorption is not assumed by 

 any special organ, unless rhizoids sometimes absorb ; but 

 true roots are complex in structure and contain vessels. 



(3) The stem. — In most of the Ferns the stem is sub- 

 terranean and dorsiventral (Fig. 118), but in the "tree 

 ferns " of the tropics it forms an erect, aerial shaft bearing 

 a crown of leaves (Fig. 120). In the other groups of Pteri- 

 dophytes there are also aerial stems, both erect and pros- 

 trate. The stem is complex in structure, the cells being 

 organized into different " tissue systems," prominent among 

 which is the vascular system. These tissue systems of vas- 

 cular plants are described in Chapter XV. 



The appearance of the vascular system ■ in connection 

 with the leafy sporophyte is worthy of note. The leaves 

 are special organs for chlorophyll work, and must receive 

 the raw material from air and soil or water. The leaves 

 of the moss gametophyte are very small and simple affairs, 

 and can be supplied with material by using very little ap- 

 paratus. In the leafy sporophyte, however, the leaves are 

 very prominent structures, capable of doing a great deal 

 of work. To such working structures material must be 

 brought rapidly in quantity, and manufactured food ma- 

 terial must be carried away, and therefore a special con- 

 ducting apparatus is needed. This is supplied by the vas- 

 cular system. These vessels extend continuously from root- 

 tips, through the stem, and out into the leaves, where they 



