412 XXX. THE SEED OF ANGIOSPERMS. 



As section-cutting of ripe seeds is difficult, we can, so far 

 as informing ourselves as to the position and structure of the 

 embryo is concerned, prepare sections from seeds which are not 

 quite ripe, and far softer, and only study the testa upon fully- 

 ripe seeds. 



Development of Embryo. Now let us go back to younger 

 stages, and at first lay the entire ovule in potash. These ovules 

 can be best obtained by opening the ovary in its entire length, 

 and removing the ovules from each half with the scalpel. The 

 ovules, almost to the fully-ripe stage, can be made so far trans- 

 parent that we can readily observe the position of the embryo. 

 In potash the embryo becomes a beautiful green ; this arises from 

 the swelling of the starch-grains, so that the chlorophyll grains 

 become visible. Proceeding to progressively younger ovules, 

 we see that the embryo (and at first especially its cotyledons) 

 becomes continually shorter. It withdraws farther and farther 

 from the lower, outward-bent, half of the embryo-sac. Ovules 

 from ovaries which, without stalk, measure about ^ inch in 

 height, show the embryo as a small body of cordate form. The 

 two divaricating anterior protuberances are the rudiments of the 

 cotyledons. As we trace back the various developmental stages 

 of the embryo, we can at the same time determine that endosperm 

 is formed only at the two ends of the embryo-sac, and appears, 

 especially at the chalazal end, as a small green-coloured mass 

 of tissue. This endosperm is displaced, and absorbed, by the 

 growing embryo, with the exception of the outermost layer, 

 which persists as an aleurone layer. We can also see that the 

 spermoderm is derived from the two cell layers of the outer 

 integument, while the cells of the inner integument are stretched 

 and ultimately crushed. If immature seeds of various ages are 

 laid upon an object-slide in a drop of potash, and covered with 

 a cover-glass, and then subjected to carefully- controlled pressure 

 with the handle of the needle-holder, the ovules w r ill burst, and 

 the often uninjured embryos will come out. This process is effec- 

 tive with embryos from a very young state up to those that are 

 well-nigh fully formed. 



In order to inform ourselves as to the structure of the egg- 

 apparatus in the ovule at the receptive period, we must have 

 recourse to alcohol material, studying ovules of various ages, 

 which we make transparent to the desired degree by careful 

 addition of potash. We can thus state the presence of two 



