gametophyte and { absent) surrounded by coats = Seed. new growth of old{ sporophyte { Young sporophyte surrounded by remnants of gametophyte and new parts of old sporophyte (remains of endosperm and of nucellus, and ovular coat) = the seed. CHAPTER XXXVII. MORPHOLOGY OF THE NUCLEUS AND SIGNIFICANCE OF GAMETOPHYTE AND SPOROPHYTE. =676.= In the development of the spores of the liverworts, mosses, ferns, and their allies, as well as in the development of the microspores of the gymnosperms and angiosperms, we have observed that four spores are formed from a single mother cell. These mother cells are formed as a last division of the fertile tissue (archesporium) of the sporangium. In ordinary cell division the nucleus always divides prior to the division of the cell. In many cases it is directly connected with the laying down of the dividing cell wall. [Illustration: Fig. 402. Forming spores in mother cells (Polypodium vulgare).] [Illustration: Fig. 403. Spores just mature and wall of mother cell broken (Asplenium bulbiferum).] =677. Direct division of the nucleus.=—The nucleus divides in two different ways. On the one hand the process is very simple. The nucleus simply fragments, or cuts itself in two. This is direct division. =678. Indirect division of the nucleus.=—On the other hand very complicated phenomena precede and attend the division of the nucleus, giving rise to a succession of nuclear figures presented by a definite but variable series of evolutions on the part of the nuclear substance. This is _indirect division_ of the nucleus, or _karyokinesis_. Indirect division of the nucleus is the usual method, and it occurs in the normal growth and division of the cell. The nuclear figures which are formed in the division of the mother cell into the four spores are somewhat different from those occurring in vegetative division, but their study will serve to show the general character of the process. =679. Chromatin and linin of the nucleus.=—In figure 404 is represented a pollen mother cell of the May-apple (podophyllum). The nucleus is in the resting stage. There is a network consisting of very delicate threads, the _linin_ network. Upon this network are numerous small granules, and at the junction of the threads are distinct knots. The nucleolus is quite large and prominent. The numerous small granules upon the linin stain very deeply when treated with certain dyes used in differentiating the nuclear structure. This deeply staining substance is the _chromatin_ of the nucleus. [Illustration: Fig. 404. Pollen mother cell of podophyllum, resting nucleus. Chromatin forming a network.] [Illustration: Fig. 405. Spirem stage of nucleus. _nu_, nuclear cavity; _n_, nucleolus; _Sp_, spirem.] [Illustration: Fig. 406. Forming spindle, threads from protoplasm with several poles, roping the chromosomes up to nuclear plate. (Figures 404-406 after Mottier.)]