314 STRUCTURE OF OPHIOGLOSSALES 



and Botrychium, are of common occurrence (Fig. 215). They 

 are of unusual interest because they present many features sug- 

 gestive of the liverworts and also of the more specialized ferns 

 and seed plants. This remark is not intended as necessarily 

 implying relationship between the three groups, but that analo- 

 gous structures have arisen in each, due either to inheritance 

 from allied ancestors or to the operation of stimuli upon plants 

 that are only slightly or not at all related. The sporophyte 

 consists of a short stem with thick fleshy roots that are associated 

 with mycorrhiza. The leaves are simple or divided and usually 

 appear singly, ensheathing the apex of the stem which always 

 remains in the soil. One of the most remarkable features about 

 these ferns is the development in the stems, roots and leaves 

 of the same tissues that we have noted in the higher seed plants. 

 The vascular bundles have the same origin as in the seed plants. 

 The woody portion (xylem) of the conducting system appears as 

 a solid strand in the center of the very young stem and this is 

 surrounded by the cellulose part (phloem) of the system. Early 

 in the development of the stem delicate cells (pith) replace the 

 central cells of the xylem so that the conducting system now 

 appears as a hollow cylinder. As the stem elongates this cylinder 

 is broken into segments by certain strands from it passing out 

 into the leaves. The spaces formed by the departure of these 

 strands are termed foliar gaps. The segments of the con- 

 ducting system are termed the vascular bundles and since the 

 phloem is outside the xylem they are known as collateral bundles. 

 In some cases these bundles develop a cambium which adds new 

 cells to the xylem and phloem so that the arrangement of tissues 

 in the stem is strikingly suggestive of the dicotyledonous stem 

 with its pith; concentric rings of xylem, cambium and phloem; 

 rays; cortex; cork; etc. (Fig. 216, 40). It is noteworthy also 

 that the cells added to the xylem by the cambium resemble those 

 appearing in the pines. The stem structure of some of the 

 simpler representatives of the next order resembles that of the 

 Ophioglossales and it has been suggested that the ancestors of 

 these two lines of Pteridophyta may have been the starting point 

 of the seed plants. 



