INTRODUCTION. 49 1 



elongated and more slender, either entirely naked or bearing one or two small leaves or 

 Bracteoles. This part of the axis is the Peduncle ; if it is very short, the flower is said to 

 be sessile. No shoots usually arise from the axils of the floral leaves, even when they 

 are produced in all the other leaf-axils of the plant ; there occur, however, abnormal 

 cases (which are not very uncommon) of axillary branching or prolification even within 

 the flower. 



The male spores or Pollen-grains are equivalent to the microspores of the higher 

 Cryptogams, and arise in receptacles corresponding to the sporangia in those plants, 

 which may be termed in general Pol/en-sacs. These are at first solid masses of tissue 

 in which, as in the sporangia, an inner mass of cells becomes differentiated into the 

 mother-cells of the pollen-grains (at first by more vigorous growth of the single cells), 

 while the surrounding layers of tissue become developed into the wall of the pollen-sac. 

 [The mother-cells of the pollen-grains are derived from one or more hypodermal cells 

 constituting the arcbesporium, which is invested by a layer of peculiar cells, the tapetum : 

 the tapetal cells are derived either entirely from the archesporium, or entirely from 

 the tissue of the anther, or partly from the archesporium and partly from the tissue 

 of the anther ^] It has already been mentioned that the mother-cells of the pollen 

 become separated and detached from the tissue (though this rule is subject to ex- 

 ceptions), and then produce the pollen-cells by division into four after actual bipartition 

 or at least an indication of it. A special description of these processes will be given under 

 the heading of the separate classes ; at present we must however premise a few facts 

 relative to the morphological nature of the pollen-sac. Like the sporangia of most 

 Vascular Cryptogams, the pollen-sacs of Phanerogams are usually products of the leaves, 

 which however mostly undergo in this case a striking metamorphosis, remaining much 

 smaller than all the other leaves. A leaf which bears pollen-sacs may be termed a 

 Staminal Leaf or Stamen ; the most recent researches have, however, shown cases in 

 which the pollen-sacs arise on the elongated floral axis itself, as Magnus has illustrated 

 in the case of Naias, Kaufmann in Casuarina, and Rohrbach in Tj/pba ; in these cases it 

 is still doubtful whether the pollen-sacs may not be the only surviving portions of 

 otherwise completely abortive staminal leaves ^. In the Cycadeae the pollen-sacs grow 

 singly or in groups on the under side of the relatively large stamens, often in large 

 numbers, resembling in position the sporangia on Fern-leaves. In the Coniferae the 

 stamens have still more lost the appearance of ordinary leaves ; they remain small, and 

 form several or only two relatively large pollen-sacs on the under side of the lamina 

 which is still distinctly developed. In Angiosperms the stamen is usually reduced to a 

 slender weak and often very long stalk called the Filament, bearing two pairs of pollen- 

 sacs at its upper end or on both sides beneath the apex, which are included as a whole 

 under the term Anther; the anther therefore usually consists of two longitudinal halves, 

 united and at the same time separated by a part of the filament termed the Connectinje. 

 The two pollen-sacs of each half of the anther are contiguous throughout their length, 

 and frequently both halves of the anther are in close apposition. The separate pollen- 

 sacs then appear as compartments of the anther, which is in this case quadrilocular, in 

 contrast to those anthers (of rare occurrence) in which each half contains only a single 

 pollen-sac, and which are therefore bilocular. 



The female spore or Embryo-sac, the analogue of the macrospore, is usually derived 

 from a hypodermal cell of the nucellus of the ovule, which must be regarded as the 

 archesporium, the ovule itself corresponding to the macrosporangium of the hetero- 

 sporous Vascular Cryptogams ^. The nucellus is a small-celled mass of tissue of usually 



^ [Warming, Unters. iib. Pollenbildende Phyllome und Caulome, in Hanstein's Bot. Abhandl. II. 

 1870: also Goebel, loc. cit.'] 



^ [For instances of the production of pollen-grains in abnormal positions, even in ovaries or in 

 the ovules themselves, see Masters, Vegetable Teratology, Ray Soc. London 1869, pp. 182-188.] 



^ [See Strasburger, Angiospermen und Gymnospermen, 1879 ; and Goebel, loc citJ] 



I. 



