1909] 



LEAVITT—HOMOEOSIS IN PLANTS 



59 



interior of a Habenaria root. It is perfectly clear that the form- 

 giving agencies which shape the leafy shoot in this species have been 

 set to work upon tissues which up to a certain point have been domi- 

 nated by other forces. There is nothing new except the extraordinary 



morphogenetic sequence. In this instance, morphic translocation 

 has become habitual, or normal. 



(b) In Phyllonoma ruscifolia of Mexico the flowers are regularly 

 produced from the upper surface of leaves, near the apex (fig. 17). 

 There is no adnation in this case, for the anatomy 

 of the blade below the inflorescence and of the 

 petiole shows no addition to the normal vascular 

 structure, indicating any concrescence. Here 

 again an abrogation of ordinary 



mor 



become 



in the development. 



(c) The specific form of the vegetatively 



derived 



sidered homoeotic: 



in some 



exam 



must be con- 



Opuntia 



vulgaris, described and figured by Ganong 

 At the time when fertilization should take place, 

 the egg cell, according to Ganong's observation, 

 has become disorganized. The place of the 



Fig. 17. — Leaf of 



normal embryo is taken by several 



budding in from the nucellus. The noteworthy ph y ll ° noma 



rusci- 



apogamous 



form of the 



is 



their 



folia. 



Here is a 



homoeosis which has become established and provides a regular 

 means of propagation for the species. A considerable number of 

 such cases is known. 



« is natural to imagine that conditions in the embryo sac deter- 



lomoeosis; yet it is not at 

 I the assumption of form. 



mine 



In a 



sac. 



form 



ed by myself the embryos or 

 Spiranthes cernua of the meadows 



upland 



seed. 



eryw 



has a normal development. In the variety 



Having followed the development with care, I find that the 



40 Bot. Gazette 25 :22I . l8l) 8. 



