PARTS OF THE EMBRYO PLANT. 283 



on the other hand, exhibit a marked separation of parts in their 

 embryo, the former having one cotyledon, and hence being mono- 

 cotyledonous (/to'oj, one); the latter two, and hence dicotyledonous (5u, 

 twice). Thus, the whole vegetable kingdom is divided into three 

 grand classes by the nature of the embryo. Fig. 499 represents a 

 monocotyledonous embryo, with its cotyledon, c ; while figs. 500 and 

 501, exhibit a dicotyledonous embryo, with its cotyledons, c c. 



591. The Spore of acotyledonous plants (fig. 498) is a cellular 

 body, from which a new plant is produced. Germination takes place 

 in any part of its surface, and not from fixed points. Some consider 

 it as produced independent of any process of fertilization, others con- 

 sider the union of two kinds of cells as necessary for its formation. 

 When formed, it sometimes presents filaments or vibratile cilia on its 

 surface (figs. 431-434), by means of which it moves about in fluids like 

 some of the Infusoria. When it germinates, these cilia disappear. 

 Sometimes spores are united in definite numbers, as in fours, sur- 

 rounded by a cellular covering, or perispore (vs^l, around, and avo^d, 

 a spore), or sporidium, and thus forming the reproductive body called 

 a tetraspore (rg^a?, four), which is common in Algas. 



592. Embryo. In the embryo or corculum (corculum, a little heart), 

 the first part formed is the axis, having one of its extremities turned 

 towards the suspensor, and the other in the opposite direction ; the for- 

 mer indicating the point whence the young root or radicle is to pro- 

 ceed, and the latter that whence the leafy stem is to arise. As the 

 first leaves produced are the cotyledons, this stem is called the cotyle- 

 donary extremity of the embryo, while the other is the radicular. 

 The radicular is thus continuous with the suspensor, and consequently 

 points towards the micropyle (fig. 494 A), or the summit of the nu- 

 cleus, an important fact in practical botany; while the cotyledonary, 

 being opposite, is pointed towards the base of the nucleus or the 

 chalaza (fig. 494 c). Hence, by ascertaining the position of the micro- 

 pyle and chalaza, the two extremities of the embryo can in general 

 be discovered. In some rare instances, in consequence of a thicken- 

 ing taking place in the coats of the seed, as in Ricinus (fig. 483), 

 and some Euphorbiacese, there is an alteration in the micropyle, so 

 that the radicle does not point directly to it. 



593. The part of the axis which unites the radicle and the cotyle- 

 don or cotyledons, is denominated caulicule or tigelle (figs. 499 t, 

 501 t). This is sometimes very short. From the point where the 

 cotyledons are united to the axis, a bud is developed (in the same 

 way as from the axil of leaves); this bud contains the rudiments of 

 the true or primordial (primus, first, and or do, rank) leaves of the plant, 

 and has been called plumule or gemmule. This bud may be seen 

 usually lying within the cotyledons. Thus, in fig. 501, the embryo 

 of the Almond exhibits the gemmule, g, lying on one of the cotyle- 



