THE EMBRYO 



45 



examined the two xylem masses were unequal in size, the larger showing about 

 half a dozen tracheids in cross-section, the smaller two or three. Whether the two 

 protoxylems are ultimately joined by intermediate tracheal tissue, so as to form a 

 continuous plate, as in the older roots of the adult plant, can not now be stated, but 

 in no cases examined was this true, and it is not unlikely that in the primary root 

 the two xylem masses are permanently separated. The cells of the foot, as usual, 

 are more or less papillate where they are in contact with the tissue of the gameto- 

 phyte. They early become infected with the endophyte, which probably makes its 

 entrance from the prothallial tissue, and not from the outside. This point, however, 

 is not perfectly clear. The infected area follows the growth of the young root, but 

 leaves the apical tissues free. 



In 0. moluccanum the leaf is the first part of the young sporophyte to develop. 

 In the larger embryos the leaf forms a conical body, merging into a nearly globular 

 basal portion, which is the foot, and within this, probably near the junction of the 

 epibasal and hypobasal halves of the embryo, the apical cell of the root is developed. 

 The leaf now shows a definite apical cell, triangular in section and exhibiting a 

 regular segmentation. The inner cells of the segments form the axial strand of 

 tissue, which is continued through the embryo into the root. The limits of the two 



Fig. 27. 



A. Section of petiole of cotyledon of 0. moluccanum, 



B. Section of young root. 



C. Vascular bundle of median region of young sporophyte. 



D. Vascular bundle of primary root. 



E. Vascular bundle of primary root of 0. pendulum; en, endodermis. 



primary organs, the leaf and the root, remain quite indistinguishable. The central 

 region, which remains surrounded by the prothallial tissue, is somewhat larger in 

 diameter and the whole of this functions as a foot, although it is composed in part 

 of tissue belonging to the root and the leaf. The young leaf elongates rapidly after 

 it has ruptured the calyptra, and its apex begins to widen out, but still shows a 

 single apical cell. As the upper part widens out there is a division of the original 

 vascular cylinder, and there is developed within the leaf a reticulate system of 

 vascular bundles or veins (fig. 22, D, E). 



As already indicated, the vascular strand of the young sporophyte is continuous 

 through the cotyledon and root, and sections at different points show essentially the 

 same structure. The petiole of the cotyledon (fig. 27, A), which is traversed by two 

 large lacunae, shows that the axial bundle is decidedly collateral in structure. The 

 xylem consists of a group of about half a dozen tracheids at the inner limit of the 

 bundle, and no endodermis can be recognized. As a section of the bundle is made 



