146 EMBRYOGENESIS IN PLANTS 



anterior-inferior octants: it does not begin from the three-sided cell 

 in one of the octants, with a concomitant suppression of the other. 

 The root apex, which is formed precociously, originates from a single 

 octant in relation to the normal pattern of segmentation. The shoot 

 apex, like the leaf, is not formed by the growth of the three-sided cell 

 in one of the anterior-superior octants, as has been generally thought : 

 the apical initial is formed by the characteristic enlargement of one of 

 the cells close to the median wall. This cell may be in one or the other 

 of the two octants. It remains rather inconspicuous until the second 

 leaf and the second root initials are becoming differentiated. The 

 initial of the second leaf, which Vladesco considers to be independent 

 of the shoot apex, and to pertain to the embryonic tissue mass, may 

 occur in the same octant as the shoot apical cell, or in the other anterior- 

 superior octant. The foot, which soon consists of enlarged vacuolated 

 cells, does not only consist of the posterior-superior quadrant, but may 

 also comprise some of the tissue derived from segments of the anterior- 

 superior quadrant, as in Pteridum aquilimmi (Hofmeister, 1851, 1857). 

 Vladesco summarises the facts by saying that the octants as such have 

 no organogenic significance. 



As the embryo continues to enlarge within the protective sheath 

 afforded by the venter, prevascular tissue becomes differentiated, a 

 strand being typically observed between the leaf tip and root tip, this 

 being conjoined with a strand which originates below the shoot apex. 

 The first root is usually diarch, while a cross-section of the embryonic 

 shoot about the level of the foot would reveal a small protostele. In 

 many, though not in all, leptosporangiate ferns, there is a close corres- 

 pondence between the number of leaves and of roots, one root being 

 typically formed at the base of each leaf. This has been interpreted by 

 Vladesco as giving strong support to the phyllorhize theory of Chau- 

 veaud(1921). 



Phyllorhize Theory. A consideration of the later stages of the 

 embryonic development leads to an analysis of the organisation of the 

 young sporophyte planthng and its further orderly development. 

 These studies have led to the formulation of conceptions such as that 

 of the phyllorhize, a term used by Chauveaud, though various phytonic 

 theories of plant construction have long been entertained (Wardlaw, 

 1952). Chauveaud recognises the spherical multicellular fern embryo 

 as a meristematic or embryonic tissue mass, from which two rather 

 different organs may be formed, namely the phylle (having the nature 

 of a foliar organ) and the rhize (having the nature of a root or radical 

 member). These two organs together constitute a unit, designated as the 

 phyllorhize, which is held to be fundamental in the construction of 

 vascular plants. At the point of union of these units in the first 



