132 ANATOMY. 



anastomose into a network, and tlie blade, instead of being entire, as is usual in this 

 class, may be lobed {Arum). On the other hand, some dicotyledons occur with 

 parallel and simple nerves ; but these exceptions do not invalidate the general rule 

 indicated above. In all cases of determining the class of a plant, the examination 

 of the nerves must be supplemented by that of the fibro-vascular bundles of the 

 stem, -which are symmetrically arranged in dicotyledons (fig. 685) ; and dispersed 

 without order, though more closely packed towards the circumference, in monocoty- 

 ledons (fig. 691). 



Buds.— The bud (fig. 696, b) appears at first under the bark as a cellular point 

 continuous with the extremity of a medullary ray ; it soon pushes through the 

 bark, and forms a tumour on the stem, when its cellular tissue becomes organized 

 into fibres and vessels communicating with those of the stem ; the medullary sheath, 

 however, of the young branch is closed at first, and does not communicate with the 

 medullary ray of the axis from which it emanates. 



Sepals. — The anatomical structure of these organs completes the analogy between 

 them and leaves. The nerves of the sepals are bundles of tracheae and fibres, 

 parenchyma is spread out between them, and their surfaces are covered by an 

 epidermis, of which the upper presents more stomata than the lower. As with the 

 leaves, the nerves of the sepals are usually parallel and simple in monocotyledons, 

 branched and anastomosing in dicotyledons. The sepals first appear as small 

 cellular papillae, connected at the base by an annular disk referable to the recep- 

 tacle : their tips are free in both the monosepalous and polysepalous calyx ; it 

 is onlj' later that the calycinal tube appears. Vascular bundles are gradually 

 formed in the sepals as in the leaves. 



Petals.— The coroUine leaves have often, like ordinary leaves, a petiole, which 

 is called the claw. When this is present, the fibro-vascular bundles traverse 

 its entire length, and only separate to form the nerves of the blade ; these 

 nerves, usually dichotomous, are composed of tracheae and elongated cells ; the 

 parenchyma which fills their interstices is formed of a few layers of cells, covered 

 by an epidermis presenting very few stomata on the upper surface only, or none 

 at all. 



Very young petals, like sepals, appear as cellular papillae ; but in petals these 

 soon dilate, and form dark or light- green disks, which at a later period always 

 change colour. Although the petals are placed below the stamens on the floral 

 axis, they generally expand later, as if they had been developed later, which is not 

 the case. 



In a monopetalous corolla, the torus is raised above its ordinary level so as to 

 form a little circular cushion which connects the leaves to which it gave birth, and 

 the segments of the corolla appear as projections upon this cushion. 



Finally, whether the corolla be monopetalous or polypetalous, its petals are 

 developed like ordinary leaves ; the tip and base are first formed, and the develop- 

 ment takes place towards the central veins from below upwards, from above down- 

 wards, and laterally. 



Stamens. — The complete stamen consists o£ filament, connective, anther, audpollen : 



