THE ANGIOSPERMAE 



H95 



The presence or absence of endosperm has a certain systematic impor- 

 tance, since it may be a family characteristic. Thus, Umbelhferae and 

 Rubiaceae have always a well-developed endosperm and Papilionaceae are 

 almost entirely devoid of it, though there may be some remains around the 

 radicle. In most families, however, it is at most a generic character. 



Where endosperm is lacking, or only weakly developed, it may be replaced 

 or reinforced by perisperm, a tissue which apparently is always derived 

 from the nucellus. The latter usually disappears during seed ripening but 

 in these cases it enlarges to form a reserve tissue which has the same 

 character as true endosperm. Perisperm forms the principal storage tissue 

 in seeds of Piperaceae and Zingiberaceae and in Cojfea the hard substance 

 of the seed, from which the beverage is made, is perisperm, the endosperm 

 being no more than a papery membrane. Since the perisperm belongs to 

 the tissues of the mother plant this is an important consideration to those 

 engaged in coffee breeding. 



Seeds of families belonging to the Caryophyllaceae and to the obsolete 

 group of Curvembryae (Bentham and Hooker) which included Chenopo- 

 diaceae, Amarantaceae, Nyctaginaceae and Phytolaccaceae, have the 

 common character of a curved embryo surrounding a mass of reserve tissue, 

 which originates as a massive thickening of the nucellus on the concave 

 side of the embryo sac and later of the embryo. This is a perisperm, but 

 in members of the Butomaceae and Alismaceae, which also have curved or 

 folded embryos, there are similar ingrowths which come from the funicles 

 and these form no reserves and are generally obliterated in the ripe seeds. 



The endosperm at first surrounds the very young embryo, but in many 

 seeds with peripheral embryos, such as Gramineae, the endosperm on the 

 outer side of the embryo disappears, so that the subsequent development 

 is all unilateral. Not all cases of unilateral embryos arise in this way. In 

 Polygonum aviciilare, for example, the young embryo shows unilateral growth 

 from an early stage and the embryo follows one of the angles of the tri- 

 angular seed and adapts itself to its curvature. 



The food reserves in the endosperm may be starch, oils or proteins or 

 combinations of these with or without mineral crystals. The cell walls are 

 sometimes amyloid and sometimes have thick secondary deposits of poly- 

 uronides (hemicelluloses), which undergo hydrolysis at germination like 

 other reserve materials. Amyloid walls give a blue colour with iodine alone. 

 They occur in Tropaeolum, Liliaceae, Amaryllidaceae, etc. The thick- 

 walled endosperms in which the thickening material consists of deposits, 

 sometimes massive, of hemicellulose, contain no starch and may be ex- 

 ceedingly hard, as is well seen in Palmae. The stone of the Date is one well- 

 known example, another is the vegetable ivory formed by the endosperm 

 of Phytelephas macrocarpa, a. tropical American Palm, which is used princi- 

 pally for making buttons and collar studs (Fig. 1369). The combination of 

 oil and starch together in the same cells is uncommon, though it occurs in 

 CeratophyUiim, but it is not rare to find oil and starch both present in the 

 endosperm, usually in inverse proportions. 



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