ANGIOSPERMS. 



557 



really bilocular anthers of Asclepiadeae and the octilocular ones of many Mimoseas. 

 Sometimes again the anther-lobes open at the apex by a pore which results simply 

 from the destruction of a small portion of tissue at this spot (Hofmeister). In other 

 respects we still want a detailed and comparative investigation of these processes, 

 which are very various and of great physiological importance; only the additional 

 remark need be made here, that it is very important from a systematic point of view 

 whether the anthers open inwards towards the gynaeceum (introrse), or outwards 

 (extrorse), the difference depending on the position of the suture and hence on that of 

 the pollen-sacs on the inner or outer side of the filament. 



In several families of Monocotyledons and Dicotyledons more or less con- 

 siderable deviations ^ occur from the course of development of the pollen and 

 from its final structure which has been here described. Naias and Zostera deviate 

 only to this extent, that no thickening of the wall of the mother-cells takes place, 

 and that the pollen-cells themselves are very thin-walled, acquiring in Zostera a very 

 strange appearance from assuming, instead of the ordinary rounded form, that of long 

 thin tubes lying parallel to one another in the anther. The deviations are more 

 considerable in the formation of compound pollen-grains. The origin of these is 

 either that only the four daughter-cells (pollen-cells) of one mother-cell remain 

 more or less closely united, like the pollen-tetrads (four-fold grains) of some 

 Orchideae, Fourcroya, Typha.^ Anona, Rhododendron, &c.; or the whole product of 

 one primary mother-cell remains unseparated and forms a mass of pollen consisting 

 of eight, twelve, sixteen, thirty-two, or sixty-four connected pollen-cells, as in many 

 Mimosese and Acaciese^. In these cases the cuticle or extine is more strongly 

 developed on the outer surface of the daughter-cells lying at the circumference of 

 the mass, and covers the whole as a continuous skin ; while only thin ridges of the 

 cuticle project from this skin inwards between the separate cells. In the various 

 sections of Orchideae every gradation occurs from the ordinary separate pollen-grains 

 of Cypripedium, through the four-fold grains of Neotiia, to the Ophrydeae, where all 

 the pollen-grains which are formed from each primary mother-cell remain united, 

 and thus a number of pollen-masses lie in one pollen-sac ; and finally to the Pollinia 

 of the Cerorchideae, where all the pollen-grains of a pollen-sac remain united into 

 a cellular mass. In this case, as in the Asclepiadeae with only bilocular anthers, 

 where the grains of each pollen-sac are firmly united by a waxy substance, it is 

 obvious that the pollen cannot be dispersed, nor can the pollen-masses fall out spon- 

 taneously from the anthers; but the flower is provided with very peculiar con- 

 trivances by means of which insects in search of honey extract from the pollen-sac 

 the pollinia or the masses of pollen which are glued together, and again get rid of 

 them on to the stigmas of other flowers of the same species (see Book III on Sexual 

 Reproduction). 



The Female Sexual Organs or Gynceceum ^ (Pistil) of the flowers of Angiosperms 



* In reference to what follows compare Hofmeister, Neue Beitrage, pt. II. (Abhand der konig. 

 Sachs. Gesellsch. VII); also Reichenbach, De poUinis Orchidearum genesi, Leipzig 1852; and- 

 Rosanoff, Ueber den Pollen der Mimosen (Jahrb. fiir wissensch. Bot. VI. p. 441). 



2 In many Mimosece the anther is, according to Rosanoff, octilocular, two pairs of small loculi 

 being formed in each anther-lobe; the pollen-cells of each pollen-sac remain united into a mass. 



^ Compare with this Payer's view (Organogenic de la fleur, p, 725). which differs in some 

 essential points. 



