the embryo 311 



Basic Types of Early Embryos 



I. Division of the zygote transverse 



A. Division in terminal cell longitudinal 



1. Both basal and terminal cells take part in the development 



of the embryo Asterad type 



2. Basal cell plays little or no part in the development of the 

 embryo Onagrad type 



B. Division in terminal cell transverse 



1. The basal cell plays an essential part in the development of 



the embryo proper Chenopodiad type 



2. The basal cell plays no essential part in the development 

 of the embrvo proper 



a. The basal cell usuall)' forms a suspensor of two or more 



cells Solanad type 



b. The basal cell divides no further and becomes a large 

 suspensor cell* Caryophyllad type 



II. Division of zygote longitudinal Piperad type 



The Asterad type is found in many families and is characteristic of 

 most Compositae that have been studied. The Onagrad type is widely 

 distributed in angiosperm families. It is frequently called the Cnicifer 

 type, but Onagrad is preferable, because it is characteristic of the 

 Onagraceae and, in this family, is the simplest type known, with the 

 exception of the reduction (Piperad) type. Many variations of the 

 Onagrad type are recognized. The Chenopodiad type, based on Cheno- 

 podium, is known only in a few genera outside the Chenopodiaceae — 

 Polemonium, Myosotis. The Solanad type, characteristic of the Solana- 

 ceae, is found in several other families. The Caryophyllad type is widely 

 distributed in both dicotyledons and monocotyledons. Many variations 

 are described in this type, as in the Onagrad type. The Piperad type 

 stands apart from all the others, not only in the position of the wall in 

 the division of the zygote but in the reduction in the number of cells in 

 the proembryo and in the morphological structure of the mature embryo. 

 Found in only rather few dicotyledons — Piperaceae, Santalales, Balano- 

 phorales, and, apparently, Scubiosa in the Dipsacaceae — it is clearly a 

 reduction type, and, with the exception of Scabioso, the genera pos- 

 sessing it are highly specialized and reduced. As a reduction type, the 

 Piperad type has also been considered hardly a basic type, but rather as 

 "unclassified. "t 



** Suspensors consisting of a few cells may be present in this type, but those addi- 

 tional to the basal cell are derivatives of the terminal cell. 



f For further details of structure of the various types and of the varieties of each 

 type, the reader is referred to D. A. Johansen, "Plant Embryology," Waltham, 

 Mass., 1950; and P. Maheshwari, "An Introduction to the Embryology of Angio- 

 sperms," New York, 1950. 



