M REPRODUCTION IN PLANTS AND ANIMALS. 



UEP110DUCTION IN PLANTS AND ANIMALS, 



TABULAE VIEW or AKALOOIU n THE DRYEI/>IK!IT or Drrmurr CLASSBS or PI.ASTS. 



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UM ovula called the micropyle, come in contact with the on 

 the apex of the ctnbryo-wc. Within the embryo-mo are to be < 

 k thi. time threeamll cell, called embryonal verucle*. " Sc 



The biatory of the development of the ovule of Orckit Mario may 

 be taken u an example of the germ-cella of the flowering plants. In 

 thia plant the ovule apringi from a plaoenUl surface a* a tingle pro- 

 [ call, which by nubdiviaion forma at last a central cell called 

 leoa, and this become* lurrounded by a layer of cello. Tbu 

 , or central cell, become* UM embryo-aao, or germ-cell The 

 Itolleo-orlU from the anthen having fallen on the utigma, now pai* 

 low* the paaaage of the irtyle, and at lait, through a little opening in 



the ouUide of 

 to be observed 



called embryonal veniclea. " Soon after 

 tit* pollen -cell hat reached the embryo-*ac, one of the embryonal 

 veckU* begin* to enlarge, and becomen divided by a crott teptum into 

 two oelU ; and while the upper one growt out in a filamentous form 

 through the micropyle by a continued proceat of cell-division, the 

 fewer cell enlarge* and divide* repeatedly, to a* to form a cellular 

 gloouU." (Henfrey.) Tbit i* the embryo. The prolonged part 

 ahMqaeatly die* away. 



The development of the pollen-cell U more uniform in the different 

 faoiilie* of plant*, The part of the ttamen called the anther at Ant 

 ipean in the young flower-bud u a little cellular papilla. In procem 

 time thi* papilla ilividrt into two portion*. The** are the nidi- 

 ent* of the future loculi, or valvea. In each half, a single axile 

 TrrUcal column of cell* *oon become* dktingni*h*d by their greater 

 in and granular content*. In each of the** cell* the nucleus dit- 

 appnn, and it replaced by two other*; thi* being followed by a 

 division of the celt-content*, which form the primordial utricle, into 

 a new cell round each ncleua. Thi* proce** it repeated, and a mat* 

 of cell* it that formed which become the parent* .of the true sperm- 



; cell* or pollen-grain*. The walls of these parent cells now become 

 thickened, their nuclei disappear, but are replaced by four permanent 

 nuclei, which become each invested with a primordial sac. In thin 

 manner each of the parent cell* is divided into four compartment*. 

 A cellulose integument is afterwards formed over each compartment, 

 which now become the pollen-grains. Like the nucleus or embryo- 

 sac of the ovule, these pollen-grains have no further power of inde- 

 pendent development or growth, but by contact with each other the 

 embryo of the seed U produced. When the anther it fully developed, 

 the external case which contains the pollen bursts, and pollen-grains 

 are distributed upon the surface of the stigma. No sooner does the 

 pollen-grain arrive upon the stigma than it loses its spherical shape, 

 and becomes elongated, forming the so-called pollen-tube. It is thi* 

 tube which, pasting down the ttyle, becomes applied to the embryo- 

 sac, and is the cause of the development and growth of the embryo. 

 A question has however arisen as to whether the pollen-tube acts 

 dynamically upon the embryo-sac, or becomes part and parcel of the 

 new embryo. 



Schleiden maintains that if the pollen-tubes be followed into the 

 ovule, it will tie found that usually one, and rarely more, penetrates 

 the intercellular passages of the nucleus and reaches the enibtyo-sac, 



; which being forced forward, U pressed and indented, and by its folding- 

 in, forms the embryo in the firvt stage of its development A bag is 

 thus formed consisting of a double membrane, the indented embryo- 

 sac, and the membrane of the pollen-tube itself. Schleiden infers the 

 identity of the embryo and the pollen-tube from the three following 

 circumstances : 1, The constantly equal ilinuieter of the pollen-tube 

 when it is just within it. 2, The invariable clii'iiiicitl similarity of 



: their contents shown by the reaction produced by the application of 



