XVI 



POLLINATION AND FERTILISATION 



219 



(iii) In a still older specimen the cells have become so arranged that 

 the following layers can be seen (a) Dermatogen, covering most of the 

 embryo ; (b) plerome, forming the central 

 mass of embryo ; (c) periblem, between the 

 two. 



,(iv) In still older stages the cotyledons 

 will have appeared. They will be seen as 

 lateral outgrowths from the upper part of 

 the embryo. 



EXPT. 190. Collect a few inflorescences 

 of the Water Plaintain (Alisma Plantago]. 

 They are in full flower in June, July, and 

 August. The flowers are small and pink in 

 colour. Remove some of the ovules, and 

 treat with potash solution. Mount in 

 glycerine, and with the cover-glass force 

 out some of the embryos. Note 



(i) The suspensor and embryonic cell. 



(ii) The embryonic cell divides up into 

 four cells. 



(iii) The cells divide into dermatogen, 

 plerome, periblem. 



^iv) Try and make out the following 

 structures 



(a) The single cotyledon which is formed 

 from the free end of the embryo ; (b) the 

 radicle and apex of root which are formed 

 from the lower cells. 



Changes in Embryo -Sac. 



During the development of the em- 

 bryo, changes are taking place in the embryo-sac. The 

 embryo-sac nucleus (p. 203), divides up and produces a number 

 of nuclei. When hundreds of nuclei have been produced, and 

 the embryo- sac has been enlarged, cell- walls begin to be formed 

 between them. Thus, a tissue of parenchyma cells is produced 

 in the embryo-sac, in which starch and proteids in the form of 

 aleurone grains are stored up for the use of the embryo. This 

 tissue receives the name of endosperm. In Wheat, Barley, and 

 Rye, the endosperm is stored up in such quantities that it is 

 not all used up in the development of the embryo, but is 

 utilised when the seed germinates and the embryo begins to 

 grow. If the ripe seed contains endosperm it is said to be 

 albuminous. If the endosperm is all used up by the develop- 

 ing embryo, so that the embryo-sac is filled with the embryo, the 

 seed is said to be exalbuminous* Wheat, Barley, and Rye are 



FIG. 215. Young embryo of 

 the Water Plantain. C, 

 cotyledon ; v, growing 

 point. (Magnified.) (S.) 



