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A TEXTBOOK OF THEORETICAL BOTANY 



Cotyledons are usually simple in outline, even in plants with com- 

 pound leaves, which may be attributed to the growth restriction we have 

 referred to. The existence of this restriction is best shown in those cases 

 where it is only temporary. The Onagraceae provide many examples of 

 this. Clarkia (Fig. 1442), Fuchsia, and some species of Oenothera have coty- 

 ledons which are at first short, broad and sessile. Intercalary growth follows 



Fig. 1442. — Clarkia integripetala. The two cotyledons, 

 right and left, show marked intercalary growth, the 

 added portions at the base being quite different from 

 the original apical portions. {From Lubbock, "A Con- 

 tribution to our Knozvledge of Seedlings'', Vol. I.) 



germination, the cotyledons elongate and the new, basal portions have 

 many of the characters of the foliage leaves, differing from the original 

 cotyledons in outline, venation and often in hairiness. The junction of the 

 two portions is marked by a constriction. The later growth also develops 

 a petiole. In these species it is evident that the cotyledonary restriction 

 has disappeared. 



How far the cotyledonary characters, especially the growth restriction, 

 can be attributed to conditions in the embryo sac and the seed, is a matter 

 not fully explored. The shape of the cotyledons in Jiiglans is obviously 

 moulded on the wall of the pericarp, but in Geranium the rolling together 

 of the cotyledons begins in the embryo sac and is not apparently due to 

 external conditions. The inequality of the cotyledons, which is often seen 

 in curved embryos, may be due to their relative positions in the seed 

 affecting them both mechanically and nutritionally. 



The absorptive function of the cotyledons in endospermic seeds may 



