xxn.] BOTANY. 91 



embryo consists of a cotyledon or cotyledons, a 

 plumule, and a radicle; of these parts each cotyledon 

 represents a leaf, the plumule and radicle together 

 form an axis, of which the first is an ascending portion 

 and becomes a stem, the latter a descending portion, 

 giving origin to the root. The plumule is, in many 

 plants, not developed till after germination. 



There are two principal kinds of embryo amongst 

 flowering plants, the mono- and di-cotyledonous; both 

 have cotyledon, plumule, and radicle, but they differ 

 most materially in their structure and mode of 

 growth. 



133. The monocotyledonous embryo is often a 

 cylindrical body, of which the upper part is the 

 cotyledon, and usually presents a longitudinal slit or 

 depression in which the plumule lies, the lower part is 

 the short, blunt radicle. In germination the plumule 

 ascends, developing alternate, often sheathing, leaves; 

 whilst the radicle either elongates for a short time and 

 is then replaced by adventitious roots or is itself 

 entirely undeveloped, but gives 'off sheathed adven- 

 titious roots (wheat, Fig. 12). 



134. The dicotyledonous embryo is more compli- 

 cated; its two cotyledons are often very large and equal 

 and are always opposite, whilst the radicle is small and 

 often short. The cotyledons maybe thick (pea, horse- 

 chestnut, acorn), or thin (maple), flat (castor-oil), or 

 folded (mallow, mustard), or crumpled (convolvulus), 

 veined with vascular bundles or not. The cotyledons 

 may remain underground and suffer no change till they 

 shrivel or decay (pea, bean, oak), or be carried up and 

 become green leaves (mustard, Fig. n) before the 

 plumule is well developed. In germination the plumule 



