580 PHANEROGAMS. 



threads, in which currents are visible, radiate to the walls from the protoplasm 

 which envelopes the nucleus. 



[During the growth of the embryo-sac its nucleus divides, and the two new 

 nuclei travel to the opposite ends of the sac, a large central vacuole being formed. 

 Each of these nuclei divides into two, and each of these again into two, so that there 

 are four nuclei at each end of the embryo-sac. One nucleus from each end now 

 travels towards the centre of the sac, where they meet and coalesce to form the 

 definitive nucleus of the embryo-sac; these two nuclei may be termed the polar 

 nuclei. Round the three nuclei at the two ends of the sac a process of free cell- 

 formation now takes place, so that there are three cells at each end ofvthe sac. 

 The cells at its lower (chalazal) end soon become surrounded with cell-walls, and 

 constitute the antipodal cells : the cells at its upper (micropylar) end remain naked, 

 and constitute the egg-apparatus. Two of the cells of the egg-apparatus lie nearer 

 the apex of the sac than the third ; they are somewhat elongated superiorly, and 

 the nucleus lies in this elongated portion, the rounded inferior portion containing 

 a large vacuole : these cells have been termed by Strasburger the Synergidcs. In 

 many cases the elongated superior ends of these cells presents a longitudinal 

 striation (as in Gladiolus, Crocus, Santalum, Zea, Polygonum, etc.), first observed by 

 Schacht*, and termed by him the Filiform Apparatus. The third cell, which lies 

 at rather a lower level than the other two, is the oosphere : it is more or less rounded 

 in form, and its nucleus lies towards its lower end. All three are usually attached to 

 the wall of the embryo-sac. It is important to note that the nuclei of the synergidae 

 are sister-nuclei; the nucleus of the oosphere is the sister-nucleus of the polar 

 nucleus which coalesces with the polar nucleus from the lower end to form the 

 definitive nucleus of the embryo-sac. 



The following are some of the principal deviations from the series of pheno- 

 mena which have been described above. Sometimes, as in Ornithogalum nutans, 

 only one of the synergidae is present. It may be that this is due to the omission of 

 one of the divisions of the nuclei at the apex of the embryo-sac ; but, inasmuch 

 as two are generally to be found in preparations of early stages and only one in 

 preparations of later stages of development, it is possible that two are primarily 

 formed but that one soon undergoes absorption. In Sinningia, according to Stras- 

 burger, only one synergida is present in some cases, and very rarely both are absent. 

 In exceptional instances two oopheres have been found in the embryo-sac, in 

 Sinningia by Strasburger, and in Gomphrena by Fischer. This peculiar abnormality 

 appears to be the rule in Santalum album. Strasburger explains it by supposing 

 either that one of the three nuclei of the egg-apparatus has undergone division, 

 so that four cells are formed instead of three, or that the upper polar nucleus 

 becomes the nucleus of the second oosphere; in the latter case, the definitive 

 nucleus of the embryo-sac would be constituted by the lower polar nucleus alone. 

 The antipodal cells are very imperfectly diff"erentiated in the Orchids, and do not 

 become clothed with cell-walls : it is this fact, doubtless, which caused Flofmeister to 

 assert that the antipodal cells are frequently wanting in Orchids. In some cases, 



^ Jahrb. fiir wiss. Bot. I and IV, [Schacht states that in Santalum album the two synergidae are 

 separated by a septum which is formed in the apical portion of the embryo-sac ; this observation has 

 been confirmed by Strasburger (Zelltheilung und Befruchtung).] 



