Beer, Development of the pollen grain and antlier of some Onagraceae. 293 



The name of „Zwischenkörper", introduced by Fritsche, 

 has been used by Nägeli and Strasburger in describing these 

 peculiar miicüaginous discs of the Onagraceous pollen grain. 

 I shall speak of these discs as „interstitial bodies" in the 

 present paper. Three is the normal number of interstitial bodies 

 possessed by the pollen grains of all the species of Onagraceae 

 which have been examined. In a few cases, however, I have 

 noticed four or even five of these bodies whilst in others only 

 two or one interstitial body occnred (Figs. 21 and 22). An 

 interesting abnorniality has been noticed in some pollen grains 

 of this age. Instead of the single nucleus which is normal at 

 this time pollen grains have been seen which contain two nuclei: 

 a large one and a small one (Fig. 18). The case is probably to 

 be compared with the irregularities which JueP) and others 

 have described in the nuclear division of the pollen-mother-ceUs 

 of Hemerocallis fitlva and is no doubt due to one or more 

 chromosomes becoming separated from the rest and forming an 

 independent nucleus. 



There appears to be some Variation in the exact time when 

 the special-mother-cell wall breaks down and sets free the pollen 

 grains. A large number of my preparations of 0. biennis show 

 this to occur at the comparatively early age that we are now 

 considering (viz pollen 22 — 24 a across). As was remarked 

 above the iirst-formed laminae of the special-mother-cell wall 

 maintain their individuality the longest and continue to give 

 callose reactions for some time (Figs. 51 and 52). The rest of 

 the wall now forms a homogeneous mucilage filling the poUen- 

 sac and occupying all the spaces between the pollen-grains. It 

 no longer has any afiinity for corallin-soda and its reactions 

 furnish no clue to its chemical nature. 



As the pollen grains continue to develope the interstitial 

 bodies become more prominent towards the exterior, giving the 

 broader face of the grain a more pronounced triangulär outline. 



A secondary thickening layer is now formed witliin the 

 first poUen wall. 



This layer extends over the whole inner face of the first 

 membrane of the pollen grain. It runs up the sides of each 

 interstitial body as a cylindrical extension which gradually thins 

 off as it approaches the apex of the body and dies away alto- 

 gether at the summit itself (see Figs. 19 and 23). The micro- 

 chemical reactions of the thickening layer do not exactly corre- 

 spond with those which are characteristic of any of the ordi- 

 nary cell-wall components and its chemical nature niust for the 

 present be left an open question. "With a rather strong Solution 

 of lodine in potassium iodide it gives a veiy beautiful violet 

 colour but with chlor-zinc-iodine and with a calcium-chloride 

 Solution of lodine it tinges only yellow or yellow-brown. Congo- 



^) Juel, O. 11.. ,.Die Kernteilungen in den Püllenmutterzellen von 

 Hemerocallis fulva etc." (Frings. Jahi-b. f. wiss. Bot. XXX. 1897. p. 205.) 



Beihefte Bot. Centralbl. Bd. XIX. Abt. I. Heft -2. 20 



