374 MORPHOLOGY OF THE ANGIOSPERMS 



The Endosperm in the Seed. Part or all of the endosperm formed in 

 early stages of embryo development serves in nutrition for the embryo 

 as it matures — both before the seed is ripe and, in many taxa, during 

 after-ripening also. Seeds are commonly described as albuminous, with 

 endosperm, and exalhuminous, without endosperm, but no line can be 

 drawn between these types, because small amounts of endosperm are 

 easily overlooked. Seeds described as exalhuminous, as in Pyrus, often 

 have small amounts of endosperm. Although endosperm is usually 

 formed at time of fertilization, the seeds of many families are described 

 as without endosperm — Compositae, Geraniaceae, Cruciferae, Cucurbita- 

 ceae, Myrtaceae, Aceraceae, Helobiales; other families have genera 

 with endosperm and others without — Araceae, Leguminosae, Rosaceae, 

 Proteaceae, Betulaceae. The Proteaceae show loss of endosperm within 

 the family; the more primitive genera have a little; other genera, none. 

 The Eleagnaceae and Cactaceae have little or no endosperm. Parasites 

 and saprophytes commonly have a uniseriate layer of endosperm cells. 

 In angiosperms as a whole, albuminous seeds are more common in the 

 monocotyledons than in the dicotyledons. Where there is no endosperm 

 and the cotyledons are poorly developed, food may be stored in other 

 parts of the embryo, especially the hypocotyl. Abundant endosperm, 

 accompanying minute, undifferentiated embryos, characterizes many 

 ranalian families. ( Calycanthaceae are a prominent exception.) This 

 ranalian seed structrue is perhaps primitive for the angiosperms. 



Histologically, endosperm varies greatly in different taxa, from soft 

 and loose ("mealy") to hard and compact, with heavy cell walls. There 

 is also great variety in the nature of the stored food — starch, oil, sugar. 



The morphological nature of the endosperm has been much dis- 

 cussed. Its complex cytological make-up — with 3n nuclei — the result 

 of a gametophytic union of two female nuclei and one male, has sug- 

 gested three interpretations. One interpretation is that it is an abnormal 

 sporophytic structure, formed by the addition of a second female nucleus 

 to the union of egg and sperm nuclei. (The addition of an antipodal, 

 vegetative nucleus to this triple union is considered unimportant.) The 

 second interpretation is that the endosperm represents an abnormal 

 gametophyte, a modified part of the vegetative tissue of the embryo 

 sac, with the addition of a male nucleus. The third theory interprets the 

 endosperm as a third phase — additional to sporophyte and gametophyte 

 — in the life cycle, a phase represented in angiosperms by remnants of 

 an ancestral structure present in thallophytes, but not found in other 

 vascular plants. 



The Perisperm. The term perisperm is applied to nutritive tissue be- 

 longing to the integuments or nucellus, or to both these tissues. It may 



