ANGIOSPERMS. 



361 



the whorls also alternate with one another and with those of the perianth ; yet there 

 are many cases of deviation from this rule, often due to the abortion of single members 

 or of entire whorls, or to the multiplication of both, or to the superposition of con- 

 secutive whorls ; two or even more stamens not unfrequently make their appearance 

 close to one another instead of a single one ; such cases are often difficult to explain, 

 but they are of great value for determining natural affinities and must be examined 

 again further on. 



Development of the pollen and anther-wall 1 . The following account refers only to 

 the ordinary cases, in which the pollen is produced in four loculaments or compart- 

 ments of the anther, in other words where there are four microsporangia present, and 

 in which it forms isolated grains which fall out on the dehiscence of the anthers ; 

 some of the more important exceptions will be noticed below. 



Immediately after the leaves of the perianth or those of the innermost whorl first 

 become visible as roundish protuberances on the circumference of the torus, the rudi- 

 ments of the stamens make their appearance in similar form, but they usually grow 



n 



FIG. 287. Althaea rosca. A pollen-sac seen from the side. B transverse section of an anther-lobe showing the 

 two pollen-sacs. > the pollen-mother-cells, in A still connected together into a tissue, in B already divided each into 

 four pollen-grains, the tapetal cells. Each anther-lobe consisting of two pollen-sacs is here borne on a long 

 branch of the filament. 



more rapidly than the corolla, which often remains for a considerable time in a very 

 rudimentary condition. The young stamen which is composed of homogeneous pro- 

 tomeristem soon shows the outlines of the two anther-lobes united by the connective ; 

 the filament is at this time still very short and for a while grows slowly, but ultimately 

 before the opening of the flower it increases rapidly in length by the elongation 

 of its cells. When the protuberances which answer to the four future pollen-sacs 

 begin to be visible externally on the young anthers, the differentiation of the mother- 

 cells of the pollen from the hitherto homogeneous tissue begins in each of the protu- 

 berances. The development of the sporangia in the Angiosperms agrees entirely with 

 that of the sporangia in the Vascular Cryptogams, as for example in the Lycopo- 

 diaceae. In both cases the young anther consists of a small-celled protomeristem 



1 Nageli, Zur Entwicklungsgesch. d. Pollens, Zurich, 1842. Hofmeister, Neue Beitr. z. Kenntn. 

 d. Embryobild. d. Phanerog. I. Monocot. Warming, Unters. ii. pollenbildende Phyllome u. Caulome 

 in Hanstein, Bot. Abh. Bd. II. 1873. Engler, Beitr. z. Kenntn. d. Antherenbildnng in Pringsh. 

 Jahrb. Bd. X. 



