570 PART III. THE CLASSIFICATION OF PLANTS. 



CLASS X. DICOTYLEDONES. 



The ripe seed (Fig. 295) may be albuminous, containing a mass 

 of endosperm and a relatively small embryo, as in the Umbelliferaa 

 and Euphorbiaceee ; but sometimes the embryo is relatively large 

 and the endosperm occupies only a small space, as in the Labiatae : 

 more commonly the seed is exalbuminous, the endosperm being 

 wholly absent, and then the embryo, which has large and fleshy 

 cotyledons, fills the entire cavity of the testa, as in the Rosaceee, 

 the Leguminosae, and the Compositae. Perisperm is rarely pre- 

 sent, either together with endosperm (e.g. some Piperales and 

 Nymphaeaceaa), or alone '(Chenopodiales, Caryopbyllales). 



The embryo (see p. 443) usually has distinct members, consist- 

 ing of an axis and two opposite cotyledons ; in rare cases, e.g. 

 Coryda.lis, only one cotyledon is present, or abnormally three may 

 occur, as is occasionally the case in the Oak, the Sycamore, and 

 the Almond. In parasites and saprophytes which are devoid of 

 chlorophyll and which have very small seeds, such as Monotropa 

 and Orobanche, the embryo is quite undifferentiated, and it con- 

 sists of only a small number of cells. 



The axis of the embryo frequently persists as the main axis of 

 the plant which grows in length and produces numerous less 

 vigorous lateral shoots ; but it often happens that some of these 

 lateral branches subsequently grow as vigorously as the main axis : 

 when this is the case, and when also the lower and feebler shoots 

 die off, a head, such as is common in forest-trees, is the result ; in 

 the case of shrubs, vigorous branches are formed quite low down 

 on the main stem. The branching of the stem is almost invariably 

 axillary and lateral : it is frequently monopoclial (p. 40), but in. 

 many forest-trees the stem (trunk) and branches form a sympo- 

 dium, the uppermost lateral bud growing each year in the direc- 

 tion of the main axis, which does not itself develope any further. 



When the axis of the embryo continues to be the main axis of 

 the plant, the primary root also developes greatly, and forms a 

 tap-root from which the lateral roots grow in acropetal succession ; 

 in cases in which the growth in length of the tap-root is limited, 

 numerous adventitious roots spring from its older portions ; these 

 may again give rise to lateral roots, and by a repetition of this 

 process an elaborate root-system is formed. 



The stem is almost always monostelic (see p. 152). The 

 vascular bundles of the stem are almost always conjoint, collateral, 



