THE ANGIOSPERMAE 1581 



by restriction. There are, nevertheless, important differences which in the 

 view of some, isolate them from the category of leaves; namely that their 

 origin in the embryo is different from that of all subsequent leaves, their 

 vascular connection with the axis may be quite different and they generally 

 have no axillary buds. As however the same line of argument can be applied 

 to the hvpocotyledonary stem and to the primary root, we might as well 

 conclude on the same grounds that the embryo is not a plant. 



The almost infinite variation of cotyledons (see Lubbock, " On Seed- 

 lings", 1892, passim) can be in general related to their variety of functions, 

 (i) They protect the apical bud both in the embryo and after germination. 

 (2) When thev are raised above ground they become photosynthetic and 

 assist the independent establishment of the seedling. In some tuberous 

 plants, e.g., Eranthis, the cotyledon is the only leaf produced in the first 

 year. (3) They are frequently used as storage organs for reserves of food 

 and mav be greatly thickened for this function. (4) They may act as 

 haustoria for the absorption of endospermal reserves, as we have des- 

 cribed. (5) They may act as boring organs in rising to the surface of the 

 soil. (6) In some JVIonocotyledons the middle piece is positively geotropic 

 and functions in pushing the seedlings down into the soil. 



Perhaps the most important distinction is between cotyledons which 

 are raised above the soil and become green (epigeal) and those which remain 

 below ground, usually in the seed testa (hypogeal). This serves to differ- 

 entiate two main types of germination. Hypogeal cotyledons are either 

 haustorial or else they themselves, in non-endospermic seeds, contain the 

 main reserves of food. In the latter case they are often greatly thickened, 

 the tissue consisting largely of somewhat thick-walled parenchyma filled 

 with reserve materials. Nevertheless they have stcmata and do not always 

 remain underground. The cotyledons in the non-endospermic seeds of 

 Cruciferae and some Papilionaceae, although storage organs, are epigeal 

 and become green and photosynthetic, serving a double function. This 

 change is usually, though not invariably, delayed until the reserve materials 

 in the cotyledons have been used up. 



The two kinds of behaviour, though physiologically important, have 

 no systematic significance and the one must be readily changeable into the 

 other, as is shown by the fact that closely related species of the same genus 

 may differ in their cotyledonary behaviour. Thus Phaseolus miiltiflorus and 

 Mercurialis annua are hypogeal, while P. vulgaris and M. perennis are epigeal. 



Most cotyledons are short lived. They are seldom abscissed and 

 usually wither after a few weeks. When they persist they are poorly 

 placed for assimilation, at the base of the stem, and can be of little impor- 

 tance. They increase in size during germination but they generally remain 

 small relative to the foliage leaves. This seems to be correlated with the 

 development of the plumular bud, for if this is removed the cotyledons 

 continue to grow in size and in Streptocarpus, where the apical bud is 

 naturally suppressed, the one surviving cotyledon may grow to be 30 cm. 

 long (see p. 1596). 



