360 



FOURTH GROUP. SEED-PLANTS. 



like the innermost stamens of Aquilegia, or they assume special forms, as in Cypripedium 

 (Fig. 284 s)\ in some Gesneraceae a gland-like structure, a nectary, takes the 

 place of a posterior stamen (see Fig. 352). Metamorphoses of this kind may be con- 

 sidered as the first steps in abortion, which may go so far that an empty place is 

 ultimately left in the flower where a stamen ought to have been, as in the Labiatae, the 

 near allies of the Gesneraceae, where the staminode has disappeared and there is nothing 

 in its place ; there are only four stamens instead of the five required by the plan of the 

 flower, and the first rudiment even of the fifth, the posterior stamen, is wanting, as 

 Fig. 285 shows. Instances of this kind completely justify us in assuming abortion in 

 cases also in which the absent organ does not disappear during development but is 

 wanting from the beginning, provided that a comparison of the number and position 

 of the parts in nearly allied plants gives ground for supposing that there is something 

 missing ; but it is the modern theory of descent which supplies certain grounds for 

 the assumption of abortion of this kind. 



con 



C. 



FIG. 286. Development of the microsporangia (pollen-sacs) of Angiosperms. A D Doronicunt macrophyllum. 

 A transverse section of a very young anther ; at a a cell of the periblem has divided into an inner cell a, the archesporium, 

 and an outer b, the primary tapetal cell ; con the connective. B transverse section through a lobe of a somewhat older 

 anther ; the archesporium a, or the cells formed from it are denoted by stronger outlines. Cpart of a longitudinal section 

 in which the archesporium a appears as a cell-row. D transverse section of an older anther, the cells of the primary tapetal 

 layer have divided and the inner layers which adjoin the archesporia a are becoming tapetal cells; gf rudimentary 

 vascular bundle of the connective con. E a microsporangium (pollen-sac) of an older anther of Menyanthes trifoliata in 

 transverse section ; snt pollen-mother-cells (sporogenous cell-group) surrounded by the tapetal cells t (darker) ; the wall of 

 the anther is of several layers through the division of the cells of the primary tapetal layer. F transverse section of a 

 loculament of an anther of Mentha aqitatica. ; a archesporium, t tapetal cells. After Warming. 



The number of the stamens in a flower is seldom limited to one or two ; like the 

 perianth-leaves they are generally present in larger numbers, and are then arranged 

 in the form of rosettes, either spirally or in whorls. If the perianth-leaves are 

 arranged spirally, the stamens also are generally so arranged, and in that case their 

 number is usually large (indefinite), as in Nymphaea, Magnolia, Ranunculus and 

 Helleborus, though it may be small (definite). But it is more common to find the 

 stamens in one or more whorls, and where there are several whorls the number of 

 the stamens in each is the same, and the same as that of the parts of the perianth ; 



