132 EMBRYOGENESIS IN PLANTS 



The precocious inception and development of the root in the 

 embryo of O. vulgatum may provisionally be referred to {a) its position 

 in proximity to the foot and to the prothallial sources of nutriment, 

 {b) the presence of auxin in concentrations that admit of the inception 

 of a root apex and of root growth. Whether or not a specific 'root- 

 forming substance' is involved must remain an open question. For 

 some considerable time after its inception, the root uses most of the 

 nutrients drawn in from the prothallus. As the root grows, a vascular 

 strand is differentiated behind its apex, the base of this strand being 

 in the approximately central position in the ellipsoidal embryonic tissue 

 mass at which the root apex originated. 



The delay in the organisation of the shoot apical meristem and the 

 first leaf may be provisionally attributed {a) to the deflection of the 

 main supply of nutrients into the precociously developing first root, 

 {b) to inadequate supplies of nitrogen-containing substances from the 

 saprophytic prothallus, and possibly (c) to a delay in the building up 

 of an effective concentration of some essential 'shoot-forming' sub- 

 stance in the epibasal hemisphere. This substance would be a metabolite 

 other than auxin, for, as Bruchmann has shown, some embryos have 

 already formed three or even four roots before the shoot apex has 

 become at all conspicuous. In one specimen, the first root, which had 

 attained a length of 6 cm, bore an adventitious root-bud about 0-7 cm 

 behind its apex. Such observations may support the view of Went 

 (1938) that a 'shoot-forming substance,' or caulocaline, is formed in 

 the root; Goebel (1902) also considered that a bud-forming substance 

 is normally formed in the roots of O. vulgatum, and hence root-buds 

 can be formed both in attached and detached roots (Wardlaw, 1953). 

 All such conceptions of the origin and action of single morphogenetic 

 substances are probably a vast over-simplification of the processes 

 actually involved. That some specific growth-regulating substance, or 

 some nitrogen-containing metabolite, is limiting in the embryonic 

 development of O. vulgatum seems to be fairly clear; for, as the evidence 

 shows, the first leaf is not only very slow-growing but it remains 

 rudimentary. It may here be noted, that, up to this time, the embryo, 

 as in Fig. 31e, is still attached to the prothallus, and is still free from 

 the endophytic fungus. After this stage, many of the embryos become 

 detached from the prothallus. Whether or not the greater growth and 

 morphogenetic activity of the shoot apex are associated with this 

 detachment from the prothallus, and, for some time at least, dissociation 

 from mycorrhizic nutrition, are questions that deserve fuller con- 

 sideration than they have been given. The functioning of the root in 

 direct contact with the soil and its uptake of water and salts also seem 

 likely to have important effects on growth relationships in the embryo. 



