DICOTYLEDONS. 



^35 



lie with their inner faces flat against one another (Figs. 435, 436); but are not 

 unfrequently folded or wrinkled and curved backwards and forwards (as in Theobroma 

 with thick, Acer and Convolvulaceae, &c. with thin cotyledons) ; less often they are 

 rolled spirally round one another (Fig. 434). 



The axis of the embryo beneath the cotyledons is generally elongated and 

 fusiform, and when of this shape is described in works on descriptive botany as the 

 Radicle. This fusiform body consists however in its upper and usually larger part of 

 the hypocotyledonary portion of the stem, and only the lower posterior terminal 

 piece, which is often very short, is the rudiment of the primary root (Fig. 437). The 

 rudiments of the secondary roots can sometimes be distinguished in the tissue of the 

 primary root (in Cucurbita, and according to Reinke in Impatiens). 



Fig. A-^^.—Rictnus commums ; I longitudinal section of 

 the ripe seed ; // germinating seed with the cotyledons still 

 in the endosperm (shown more distinctly in A and />), 

 s testa, e endosperm, c cotyledon, he hypocotyledonary 

 portion of the stem, w primary root, -w' secondary root, 

 X the caruncle, an appendage of the seed characteristic of 

 Euphorbiaceae. 



\r.s 



FtG. 436. — Vicia Faba: A seed with one of the coty- 

 ledons removed, c the remaining cotyledon, -w radicle, 

 kn plumule, j testa ; B germinating seed, j testa, / a por- 

 tion of the testa torn away, n hilum, st petiole of one of 

 the cotyledons, k curved portion of the axis above the 

 cotyledons, he the very short hypocotyledonary portion of 

 the axis, h the primary root, ws its apex, kn bud in the 

 axil of one of the cotyledons. 



Germination generally takes place — after the testa, or in dry indehiscent fruits 

 the pericarp, has burst from the swelling of the endosperm or of the cotyledons 

 themselves — by the elongation of the hypocotyledonary portion of the axis to such 

 an extent as to push the radicle out of the seed, the root then beginning to grow 

 rapidly and generally attaining a considerable length and forming secondary roots 

 in acropetal succession, while the cotyledons and plumule still remain in the seed 

 (Figs. 435, 436, 437). Thick fleshy cotyledons usually remain in the seed during 

 germination, finally perishing after their food-material has been consumed (as in 

 Phaseolus muUiflorus, Vicia Faba, Fig. 436, Quercus, Fig. 438). In this case the 

 petioles of the cotyledons lengthen so much that the plumule which is concealed 



