240 EMBRYOGENESIS IN PLANTS 



contributing to the lower region of the embryo, i.e. the root and root- 

 cap. The embryo now begins to elongate and an indentation appears 

 at the junction of the single cotyledon and the hypocotyl, Fig. 56k, This 

 is the shoot apex. This series is representative of Luzida and other 

 simple monocotyledonous types. In the Liliaceae, e.g. Muscari, the 

 basal cell also contributes to the lower region of the embryo. In 

 Sagittaria the basal cell does not divide: it becomes distended and takes 

 no part in the formation of the embryo proper. In the formation of the 

 latter the terminal cell undergoes two transverse divisions. The distal 

 cell then divides longitudinally into quadrants and further longitudinal 

 divisions take place in the other tiers in a basipetal sequence. Later the 

 two most distal tiers give rise to the cotyledon, the subjacent tier to 

 the shoot apex, the next two tiers to the root and root-cap, and the 

 tiers belovv' to the suspensor. In such an embryo there is evidently a 

 locus of active growth in the distal region of the embryo. Although the 

 mature embryo in the Gramineae is a complex structure, the early 

 stages are simple and like some already described. These embryos are 

 considered more fully in Chapter XIV. 



TAXONOMIC DISTRIBUTION OF THE EMBRYONOMIC TYPES 



It is of considerable interest to know to what extent a classification 

 based on embryonomic types is in accord with one based on the usual 

 taxonomic criteria. The following Table gives some indication of the 

 distribution of the several embryonomic types in different orders and 

 families. 



1 . Pi per ad type: 



Peperomia (Piperales-Piperaceae) 

 Balanophora (Santalales-Balanophoraceae) 

 Scabiosa (Asterales-Dipsacaceae) 

 Euphorbia (Euphorbiales-Euphorbiaceae) 



2. Ouagrad or Crucifer type: 



Godetia, Oenothera, Lythrum 



(Lythrales-Onagraceae and Lythraceae; some Onagraceae, 



e.g. Epilobium are exceptions) 

 Capsella, Alyssiim 

 (Cruciales-Cruciferae) 

 Myosurus 



(Ranales-Ranunculaceae) 

 Euphorbia (Euphorbiales-Euphorbiaceae) 

 Mentha (Lamiales-Labiatae) 



