134 EMBRYOGENESIS IN PLANTS 



referred to metabolic factors and relationships. In particular they 

 appear to be largely, if not entirely, related to the nutritional resources 

 which can be mobilised from a purely saprophytic prothallial system 

 on the one hand, as compared with a photosynthetic leaf and mycor- 

 rhizic root system on the other. This finding may have a more general 

 application. The 'normal' embryonic development of any species takes 

 place as it does because the embryo is developing in a closely regulated 

 and specific metabolic system. If, however, the nutrition available to 

 the embryo is modified in certain ways, the embryonic development 

 may be more or less considerably modified, but it will, nevertheless, 

 take place within the general embryogenic pattern for the species. 

 Extreme changes in the embryonic development of Datura stramomum 

 have been made by van Overbeek, Conklin and Blakeslee (1941, 1942) 

 and van Overbeek (1942) by culturing excised young embryos in a 

 medium with added coconut milk {see also Chapter XV). 



EMBRYONIC DEVELOPMENT IN OTHER SPECIES OF OPHIOGLOSSUM 



Campbell (1911, 1940) has recognised three types of embryogeny in 

 the genus Ophioglosswn, namely, (i) the O. vulgatum type as described 

 above; (ii) the O. moluccamim type as described by Mettenius (1856) 

 and Campbell (1911, 1921, 1940); and (iii) the O. pendulum type as 

 described by Lang (1902) and Campbell (1911, 1921, 1940). 



In O. moluccanum, according to Campbell, the epibasal segment of 

 the young embryo gives rise to the cotyledon, or first leaf, {leaf 1), the 

 whole of the hypobasal region being occupied by the large foot. The 

 primary root {root 1), which originates endogenously in the central 

 region of the embryo, grows rapidly downwards through the foot and 

 fastens the young sporophyte to the soil. The first leaf also elongates 

 rapidly, breaks through the prothallial tissue, and appears above 

 ground as a delicate lanceolate and presumably green leaf. At this 

 stage, the foot consists of a zone of large cells in the equatorial region 

 of the bipolar embryo which, according to Campbell, now consists of 

 the cotyledon and root only. The first leaf is initially a conical object 

 with an apical cell but later it becomes narrowly laminate and petiolate. 

 The leaf stalk merges with the root, their tissue systems being con- 

 tinuous. The shoot apex, which originates as an endogenous bud close 

 to the stele in the primary root, consists initially of a small mass of 

 active meristematic cells. Campbell states that the second leaf 

 originates independently of the shoot apex but is closely associated with 

 it. Its vascular strand becomes conjoined directly with the stele of the 

 primary root. No vascular strand can be discerned below the rudi- 

 mentary shoot apical meristem; the latter is enclosed within the sheath- 

 ing base of the second leaf, but root parenchyma may also contribute 



