POLLEN. 



ried pollen occurs within the limits of the 

 same family, and very similar pollen-grains 

 in families widely distant. There appears, 

 however, to be a certain relation within the 

 limits of genera. It may be perhaps gene- 

 rally stated that the Monocotyledons have 

 frequently one pore or furrow ; the Grasses 

 often three pores, as is the case with many 

 Dicotyledons, many of which have more, 

 while a large number of the families of this 

 division exhibit both pores and slits. As 

 microscopic objects, it is most convenient to 

 class the forms artificially, or according to 

 structure ; and we give a brief list of the 

 principal varieties arranged under this point 

 of view. 



The pollen-grains of Zostera, Zanichellia, 

 and other submerged aquatic plants, have 

 no cuticle or outer coat; all other known 

 forms possess one or more outer layers. 



A. Outer coat without furrows or pores. 



a. Outer coat granular : Strelitzia Regince, 

 Calla palustris, Crocus sativus, fyc., 

 Asarum europceum, Laurus nobilis, fyc., 

 many Euphorbiacese. 



b. Outer coat granular : Canna indica. 



c. Outer coat with cell-like reticulations : 

 Ruelliafprmosa(P\. 32. fig. 23), R. stre- 

 pens, Tribulus terrestris. 



In Periploca grceca (PI. 32. fig. 15) and 

 Apocynum Venetum\(fig. 7) grains of this kind 

 are connected in fours in one plane ; in some 

 Luzulce tetrahedrally.. 



B. Outer coat presenting longitudinal fur- 

 rows (or folds). 



* One furrow (the form of most Mono- 

 cotyledons). 



a. Outer coat slightly granular : common 

 in Monocotyledons ; among the Dico- 

 tyledons, in Myrica cerifera, Magnolia 

 grandiflora, Liriodendron tulipiferum, 

 &c. 



b. Outer coat coarsely granular or spiny : 

 Nymphcea alba. 



c. Outer coat with cell-like reticulations : 

 Hemerocallis fulva, and other Monoco- 

 tyledons. 



d. Outer coat with irregular reticulations : 

 Alstrcemeria Curtisiana. 



Among the Orchidese are found quater- 

 nate grains belonging to this group. 



[ 519 ] POLLEN. 



** Outer coat with two furrows : a rare 

 form, occurring in species of Ponte- 

 deria and Amaryllis, Tamus commu- 

 nisanAelephantipes, Tigridia pavonia, 

 Calycanthus fioridus, fyc. 



*** Outer coat with three longitudinal 

 furrows. 



a. Outer coat granular. One of the com- 

 monest forms : Quercus Robur, Viola 

 odorata (PI. 32. fig. 6). 



b. Outer coat with short spines : Cactus 

 Jlagelliformis, Viscum album. 



c. Outer coat with cell-like reticulations : 

 Statice (PI. 32. fig. 29), various Cruci- 

 ferae. 



**** Outer coat with more than three 

 furrows. 



a. Four : very rare as normal, Houstonia 

 ccerulea, Cedrela odorata; occasionally 

 occurring where three is the normal 

 number, as in Solanum tuberosum. 



b. Six : some of the Labiatae and Passi- 

 floreae (PI. 32. fig. 20), Ephedra dista- 

 chya, Heliotropium grandiflorum. 



c. A larger number of furrows : many 

 Rubiaceae; e.g. Sherardia arvensis (PI. 

 32. fig. 18). ' 



The pollen of the Pines is related to this 

 group, also that of Nymphcea Lotus, Victo- 

 ria regia, and other plants, where the fur- 

 rows or thin places occupy the greater part 

 of the wall, and the outer coat forms only 

 segmental pieces. In Mimulus moschatus 

 (PI. 32. fig. 24) a very remarkable appear- 

 ance arises from the furrows running in 

 a curved or spiral direction, and analogous 

 conditions are met with in Thunbergia alata. 



C. Outer coat with pores. 



* A single pore : Grasses, Sedges, Ty- 



pha angustifolia, Sparganium ra- 



mosum. 

 ** Two pores : Colchicum, and a few 



other Monocotyledons; also Brous* 



sonetia. 



*** Three pores. 



a. Outer coat granular : Dipsacese, Urti- 

 caceae, Onagraceae (here the pores form 

 projecting processes (PI. 32. fig. 14) ;) 

 and in Morina persica this is still more 

 the case ; Cucumis sativus. 



b. Outer coat with cell-like reticulations : 

 many Passifloreae (with large lids, P. 

 ccerulea (PI. 32. fig. 13), alata, &c.). 



