334 DICOTYLEDONES. 



Class II. Dicotyledones. 



In this class THE EMBRYO lias 2 seed-leaves, a rule from which 

 there are few exceptions (e.g. Ficaria, Cyclamen, Pinguicula, 

 certain species of Corydalis, with only 1 ; and a few, mostly para- 

 sitic forms, e.g. Monotropa, Orobanche, Pyrola, entirely without 

 cotyledons). On germination the cotyledons nearly always raise 

 themselves above the ground as green, assimilating leaves and are 

 then termed aerial or epigean, in contradistinction to the under- 

 ground or hypogean which are always buried. The structure of 

 the seed varies (endospermous or exendospermous) ; the embryo 

 may be straight or curved. In many instances the primary root 

 grows as a vigorous tap-root, with weaker branches arising acro- 

 petally (in annuals, biennials, many perennials, especially woody 

 plants) ; but in a large number of herbaceous perennials, which 

 have rhizomes, the root behaves very much as in the Monocoty- 

 ledons. The roots generally increase in thickness by means of a 

 cambium. 



THE STEM, when seen in transverse section, has its vascular 

 bundles arranged in a ring ; in reality, however, they form a 

 kind of cylindrical network in the stem ; the bundles are open, 

 and thickening takes place by means of a cambium ; annual rings 

 are formed in the perennial stems. There is a rich and very varied 

 form of branching. The two first leaves of a shoot (fore-leaves) 

 are placed nearly always to the right and to the left ; the same 

 arrangement is found in the two first leaves developed on the 

 flower-stalk, and these are, as a rule, the only two ; they are found 

 below the calyx and are usually termed the " bracteoles" It has 

 become customary to indicate the bracteoles by the letters a and /?, 

 according to their sequence of growth, and in that sense these 

 letters will be employed in the following diagrams. 



THE ARRANGEMENT OF THE LEAVES varies very much ; there is 

 also a great variety of shapes in the leaves and their venation, 

 but the linear leaves, with parallel venation, so frequent in the 

 Monocotyledons, are seldom met with, as also the large sheaths 

 (though the sheath is well developed in the Umbelliferous plants) ; 

 stipules occur much more frequently. 



THE FLOWER is most commonly cyclic, but acyclic or hemicyclic 

 forms also occur. The type which may be taken as a basis 

 consists in the majority of instances, as in the Monocotyledons, of 

 5 whorls, of which the 4 outer ones (calyx, corolla, and the 2 



