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A TEXTBOOK OF THEORETICAL BOTANY 



It will not have escaped notice that the sheathing part of the cotyledon 

 has not altered its nature with the descent to its base of the attachment of 

 the middle piece. Although now above the point of attachment to the seed 

 it is still the sheathing base of the cotyledon. Indeed it is probably wrong 

 to speak of the descent of the point of attachment. Rather has the coty- 

 ledonary sheath developed upwards, assuming negative geotropism and a 

 new function as a boring organ for the hypogeal plumule. Such an up- 

 growth can be followed ontogenetically in Tradescantia. In this genus we 



also see the beginning of a further process, namely 

 the union of the cotyledonary middle piece with 

 the hypocotyl, a union which has become com- 

 plete in Carex and Cyperus and is also found in 

 some of the Grasses. This compound axis is a 

 new structure and it is called the mesocotyl. 



The vascular bundle from the haustorium 

 enters the mesocotyl but remains distinct and 

 only joins the vascular system of the hypocotyl at 

 the base of the plumule, where it divides to 

 supply two bundles to the cotyledonary sheath. 

 In Carex it does not unite until it has travelled 

 up into the sheath and down again (Fig. 1454). 

 The cotyledonary ligule was presumably originally 

 a ring, but it seems most probable that it is now 

 represented only by the epiblast. No Grass has 

 retained a complete ligular ring, but Oryza shows 

 an approach to it. 



We may now suggest homologies for these 

 various structures in a tentative way, based on 

 the foregoing considerations. The scutellum 

 represents the haustorial part of the cotyledon. 

 So much is generally agreed. The middle piece 

 has been united to the hypocotyl to form the 

 mesocotyl. The coleoptile is the basal sheath of 

 the cotyledon, now advanced upwards in accord- 

 ance with its new function as protector and 

 leader of the plumule. It is not possible to inter- 

 pret it as the first plumular leaf, as has been 

 the sheath. C, Section attempted, because it should then be opposite 

 of the sheath, the double the cotyledon, whereas it stands on the same 

 bundle shown in black, ^jj^ ^^ ^j^^ scutellum, with its adaxial side, as 



marked by the slit opening through which the plumule emerges, facing 

 away from the scutellum. In Cyperus this is more obvious than in most 

 Grasses. The epiblast may be the remains of the cotyledonary ligule. 

 In some grasses, e.g., Zizania aguatica. Wild Rice, the epiblast is remark- 

 ably long and well developed and it may be that it has survived through 

 fulfilling a useful protective purpose (Fig. 1455). The coleorhiza is simply 



Fig. 1454. — Diagram of the 

 vasculation in the seed- 

 ling of a Carex. A, 

 Longitudinal section. 

 The haustorial portion of 

 the cotyledon is attached 

 to the side of the hypo- 

 cotyl but its vascular 

 strand descends from the 

 tip of the cotyledonary 

 sheath or coleoptile. B, 

 The double bundle in 



