310 MORPHOLOGY OF THE ANCIOSPERMS 



The long period between pollination and fertilization has been con- 

 sidered a primitive character, apparently, in part, because of the long 

 period found in some of the Amentiferae. The shortest periods are 

 characteristic of annuals — Zea and other grasses. The long periods of 

 the red oaks, contrasting with much shorter periods in other oaks, are 

 doubtless associated with the adaptation of the red oaks to the colder, 

 temperate climates. Perhaps the very long, as well as the very short, 

 periods represent specializations. 



THE EMBRYO 



The Proembryo. After fertilization, the zygote rests for a period that 

 varies greatly in different taxa — from a few hours to several weeks, 

 even over winter in some autumn-flowering plants. Development of the 

 endosperm — division of the primary endosperm nucleus — usually be- 

 gins immediately after fertilization, before division of the zygote, but 

 may accompany or, rarely, precede syngamy. 



Methods of development of the first few cells of the proembryo vary 

 considerably. Six general types are distinguished, though most of these 

 show variations that make clear distinctions among them not always 

 possible. The types are based on the position of the first cell walls in 

 the zygote and early proembryo, and on the extent and part of the 

 embryo proper developed by the first-formed cells. In five of the types, 

 division of the zygote is transverse; in the sixth type, which is rare, it is 

 longitudinal, or obliquely so. (Of the two cells formed by a transverse 

 division, that toward the center of the embryo sac is called terminal; 

 that toward the micropylar end, basal.) In the group of types in which 

 the first division is transverse, the position of the next division, which is 

 in the terminal cell, determines two subgroups. This division, like that 

 in the zygote, is either longitudinal or transverse and forms subgroups 

 based on the part played by the basal cell in continuing the develop- 

 ment of the embryo. 



The terms applied to the types of early embryogeny — to proembryo 

 stages — are based on the names of families in which members of each 

 type are found. The term proembryo is variously used. It is commonly 

 applied to the very early stages of the embryo — the first few cells — the 

 stages on which the classification below is based. But it is also often 

 used to cover all stages of the embryo while it remains radially sym- 

 metrical, before differentiation of cotyledons and axis begins. This is 

 probably the better use; otherwise, there is no term for the inter- 

 mediate stages. The following outline — in key form — is a convenient 

 summary of the six types. It does not indicate relationships among the 

 types. 



