THE EMBRYO 



339 



Division of the cotyledon into proximal and distal parts — parts that 

 differ in function and are even separated in cotyledons by fusion of the 

 middle section to the hypocotyl — is a structural result of the retention 

 of the cotyledon tip within the seed coats. Retention of the seed coats 

 in the ground involves early withdrawal of the plumule from the seed 

 coats and the penetration of the soil by the plumule tip, as the plumule 



Fig. 132. Sketches of seedlings showing variety of cotyledon form, neck not connate 

 to cotyledonary sheath, slight development of shoulders in some. A, Dracaena draco; 

 B, Arum italiciim; C, Hyacinthiis candicans; D, Iris pscudacorus. cs, cotyledonary 

 sheath; h, hypocotyl; /', /", first and second leaves; pi, plumule. (A to D, after 

 Lewin; E, Tschirch; F, Boyd.) 



elongates. The sheathing base of the cotyledon, which, in the more 

 primitive epigeal taxa, surrounds and protects the plumule, becomes a 

 more complete and effective protecting shield, the coleoptile. 



The nature of the coleoptile (pileole of some older textbooks) has 

 long been a highly controversial subject. It has been interpreted 

 as the first true leaf, the ligule of the cotyledon, a "plumular scale," the 

 sheathing base of the cotyledon, a ligular sheath, a stipular sheath 



