REPRODUCTION, VEGETABLE (VEGETABLE OVUM). 



248 



separated from each other by as many vertical 

 septa, meeting at right angles. Each corpus- 

 culum is likewise surrounded on all sides by a 

 single layer of cellules resembling pavement 

 epithelium, and exhibits in its interior a nu- 

 cleus which is usually placed at its superior 

 extremity. After some time the nucleus dis- 

 appears, and now a number of transparent 

 vesicles become visible, which accumulate for 

 the most part towards the extremities of the 

 corpusculum, 



Fig. 188. 



Corpusculum of tlie same in section, immediately be- 

 fore impregnation, 200 diam. 



Its cavity is filled with transparent vesicles, and 

 bounded superiorly by four granular cells, the 

 position and relations of which recal very forcibly 

 the arrangement which presents itself in the 

 rudimentary archegonium among the higher 

 Cryptogamia. 



104. The growth of the pollen tube, which 

 has been for many months arrested, at last re- 

 commences ; the membrane of the summit of 

 the embryo-sac at the same time becomes 

 attenuated, and immediately after is pene- 

 trated by the narrowed end of the pollen 

 tube, which is brought into immediate con- 

 tact with the summit of the corpusculum, the 

 four cells which previously surmounted it 

 having disappeared. At this time, the corpus- 

 culum exhibits in its interior, at the end oppo- 

 site the pollen tube, a single vesicle, much 

 larger than those by which it is surrounded, 

 within which is afterwards developed a se- 

 condary cell, occupying more than half its 

 cavity. This cell, which is convex above, is 

 applied by a flattened inferior surface against 

 the wall of the corpusculum. It soon divides 

 by a longitudinal septum into two, each of 

 which is nucleated. These two cells, which 

 occur throughout the Coniferae, form the 

 commencement of the suspensor. They next 

 divide by a second pair of vertical septa, at 

 right angles to the first; and in each of the 

 four cells which result, a succession of hori- 

 zontal septa are formed, by which they are 

 converted into four vertical columns inti- 

 mately united to each other. The suspensor 

 lengthens in one direction only, partly by the 



repeated division of the four inferior terminal 

 cells, partly by the interstitial growth of those 

 first formed. Soon it bursts through the 



Fig. 189. 



Corpusculum of the same, 120 diam. 



The four cells by which it is bounded superiorly 

 have disappeared. The pollen tube is still in 

 contactj by "its flattened extremity with the 

 corpusculum, and by the rest of its surface with 

 the cells of the albuminous body, a, 300 diam. 

 Earlier stage of development of lower end of the 

 same. A single germ-cell is applied to its wall. 

 b, 300 diam. Division of germ-cell into four by 

 two vertical septa, of which one only can be seeu 

 on section, c and d, 250 diam. Division of the 

 four resulting cells by a succession of transverse 

 septa. Above d are two of the numerous com- 

 plexes of cells, which at this time float in num- 

 bers in the corpusculum. (From fig. 172 to 189 

 inclusive, are after Hofmeister). 



membrane of the corpusculum at its lower 

 end, and becomes immersed in the tissue which 

 occupies the embryo sac, the cells of which, 

 at the same time, become less intimately 

 united than before. The four series of cells 

 of which the suspensor is formed, now separate, 

 and, from the terminal cell of each, the rudi- 

 ment of an embryo takes its origin. Its 

 development commences, like that of the 

 embryos of theHepaticae and of the first leaves 

 of the Ferns and Equisetacese, by the repeated 

 formation of alternately inclined septa in a 

 terminal cell ; these being followed by vertical 

 septa radiating from the axis, and, subse- 

 quently by others parallel to the external 

 surface. Of the four embryos thus formed, 

 one only advances to vigorous maturity. 



105. Phancrogamia angiospermia. The ob- 

 servations on record relating to the origin and 

 development of the embryo among these 

 plants are now so numerous, that although 

 the conditions are much more complicated, 

 and the difficulties in themselves much greater, 

 we are, notwithstanding, more competent to 

 draw our conclusions with confidence than 

 we have found ourselves to be in our previous 

 study of the Cryptogamia. Among the many 

 examples at our disposal, we select two of the 

 simplest, between which, at the same time, 

 great differences present themselves in those 

 respects in which the development is variable. 



