IX 



MARA TTIA CE.E—ISOK TA CEAC 



263 



older embryo shows an external differentiation into the first 

 leaf, stem, and root, but the foot is not clearly limited at first. 

 The basal wall separates the embryo into two re<;ions, cpibasal 

 and hypobasal. From the former the cotyledon and stem apex 

 are derived, from the latter the root and foot. 



The cotyledon arises from the anterior pair of cpibasal 

 octants, which arc in the Marattiaccai, unlike all the other Ferns, 

 turned away from the archegonium opening. In the earliest 

 stages where the cotyledon is recognisable, no single apical cell 

 could be made out, and later the growth is very largely basal. 

 At first the growth is nearly vertical, but it soon becomes 



Fig. 136. — Marattia Douglasii (Baker). A, Cross-section of the young sporophyte at the junction 

 of the cotyledon and stem ; st, the apical meristem of the stem, X215 ; B, the stem apex of the 

 same, X430; C, longitudinal section of the stem apex of a plant of about the same age, X215 ; tr, 

 the primary tracheary tissue ; r-, the second root. 



stronger upon the outer side, and the leaf rudiment bends 

 inwards. At this stage the different tissues begin to be dis- 

 tinguishable. Somewhat later the tip of the cotyledon becomes 

 flattened, and still later there is a dichotomy of this flattened 

 part which thus forms a fan-shaped lamina (Fig. 138). The 

 first tissue to be recognised is the vascular bundle, which 

 traverses the centre of the petiole and at first consists of 

 uniform thin-walled elongated cells (procambium). This forma- 

 tion of procambium begins in the centre of the embryo and 

 proceeds in three directions, one of the strands going into the 

 cotyledon, one in an almost opposite direction to the primary 



