STRUCTURE.] DICOTYLEDONOUS EMBRYO. (51 



others are naturally distant from each other without any 

 intervening substance. Some are straight, some waved, others 

 arcuate or spiral. When they are folded with their hack 

 upon the radicle, they are called incumbent ; if their edges are 

 presented to the same part, they are accumbent ; terms chiefly 

 used in speaking of Crucifers. 



Upon certain differences in the structure of the embryo, 

 modern botanists have divided the whole vegetable kingdom 

 into three great portions, which form the basis of what is 

 called the natural system. These are, 1. Dicotyledons; 

 2. Monocotyledons ; and, 3. Acotyledons. In order to un- 

 derstand exactly the true nature of the embryo in each of 

 these, it will be requisite first to describe it fully as it exists 

 in dicotyledons, and then to explain its organisation in the 

 two others. 



If a common DICOTYLEDONOUS embryo (Plate VI. fig. 2.), 

 that of the Apple for example, be examined, it will be found 

 to be an obovate, white, fleshy body, tapering and solid at the 

 lower end, and compressed and deeply divided into two equal 

 opposite portions at the upper end ; the lower tapering end 

 is the radicle, and the upper divided end consists of two 

 cotyledons. Within the base of the cotyledons is just visible 

 a minute point, which is the plumule. The imaginary line of 

 division between the radicle and the cotyledons is the cauli- 

 cule. If the embryo be placed in circumstances favourable 

 for germination, the following phenomena occur : the caulicule 

 will extend so as to separate the cotyledons from the radicle 

 by an interval, the extent of which varies in different plants ; 

 the radicle will become elongated downwards, forming a little 

 root ; the cotyledons will either elevate themselves above the 

 earth and unfold, or, remaining under ground, will part with 

 their amylaceous matter and shrivel up ; and the plumule will 

 lengthen upwards, giving birth to a stem and leaves. Such is 

 the normal or proper appearance of a dicotyledonous embryo. 



The exceptions to it chiefly consist, 1 . in the cohesion of the 

 cotyledons in a single mass, instead of their unfolding; 2. in 

 an increase of their number; 3. in their occasional absence ; 

 and, 4. in their inequality. 



