622 COTYLEDONS. 



their green blades should be exposed as much as possible to the sun, and that so 

 they should rise above other objects which might place them in the shade. The 

 accompanying figure 148 shows the most noticeable forms of cotyledons after they 

 have unfolded and spread out in the sunlight. 



When two green cotyledons are present they are usually similar in shape and 

 size, only that which has served in the seed as an absorbent organ is generally 

 somewhat smaller in the adult condition, as, for example, in the Corn-cockle, 

 Mustard, and Hemp. Frequently the limited character of the space within the seed 

 makes it necessary that one of the cotyledons should give place to the radicle, or 

 that it should only attain to an inconsiderable development, as, for example, in 

 Petiveria and Ahronia. In species of Streptocarpus belonging to the Gesneraceae 

 (see figs. 148 ^'^' ^^' ^^' ^^•), the two cotyledons have the same shape and size in 

 the seed, after they have left the seed-coat they are still entirely similar, but later 

 on the growth of one is retarded, and it dies, while the other increases to an extra- 

 ordinary degree, and develops into a green foliage-leaf lying on the ground, 22 cm. 

 long, and 12 cm. broad. Strangely enough, many species of this genus, e.g. 

 Streptocarpus polyanthus develop no other green leaves, but content themselves 

 with the development of the one cotyledon into a gigantic foliage-leaf prostrate on 

 the soil, with wliich later on the epicotyl appears to be united, and from whose 

 thick midrib it rises up as a flowering axis. 



It is without question that cotyledons which become green possess, in common 

 with other green tissues, the property of manufacturing organic materials in the 

 sunlight from the absorbed food-gases, salts, and water. As a rule chlorophyll does 

 not appear until the cotyledons have issued from the seed-coat, and have spread out 

 in the sunlight. It is, however, sometimes formed even while the cotyledons are 

 still in the seed and shrouded in darkness, as, for example, in firs and pines, in 

 maples, and some Cruciferse, in Loranthus, Mistletoe, and the Japanese Sophora. 

 Green cotyledons exhibit all the characteristics of foliage; their epidermis is pro- 

 vided with stomata, whilst palisade-cells and spongy parenchyma can usually be dis- 

 tinguished in the green tissue. Many plants, especially those which subsequently 

 develop subterranean tubers, or tuberous roots, e.g. many species of Ranunculus, 

 Monkshood, Corydalis, Eranthis, Leontice, Bunium, Smyrnium perfoliatum, and 

 Ghcerophyllum bulbosum, do not in the first year after germination go beyond the 

 formation of green cotyledons; green shoot-leaves are not developed from the bud 

 or plumule until the next year. Many plants, on the other hand, unfold green 

 shoot-leaves almost simultaneously with the cotyledons, but the cotyledons function 

 with them as foliage, and sometimes remain fresh and green until the time of 

 flowering, or even until the ripening of the fruit. Examples of these are aflforded 

 by numerous quick-growing annual weeds in our fields and kitchen-gardens 

 (e.g. FuTTiaria officinalis, Scandix Pecten-Veneris, Arnoseris pusilla, Urtica urens, 

 Adonis cestivalis). The cotyledons, in rapidly-developing annuals, sometimes attain 

 dimensions scarcely inferior to those of the green shoot-leaves. For example, the 

 cotyledons of the Gourd are more than a decimeter long, and 4-5 cm. broad. It is 



