POLLEN. 



97 



itner-cells, united together in fours, and in this condition they leave the aiither- 

 dties. These little pollen-aggregates are termed tetrads. Examples of such 

 tnts are: — the Ericaceag {Erica, Calluna, Menziesia, Andromeda), the Bearberry 

 rctostaphylos), the Strawberry Tree (Arbutus), the Alpine Eose (Rhododendron), 

 dum, Kalmia, the Cranberry and Bilberry (Vaccinium); the Epacridacese, 

 )acris and Leucopogon; many Winter-greens (Pyrolaceaa); a few Sedges (Juncus 

 ■cquinii and Luzula vernalis); finally Anona, Drimys, Jussieua. In the plants 

 it enumerated the tetrads correspond to the four pollen-cells in the comers of the 

 )ther-cell (cf. fig. 219 2); but in the Apocynaceae {Apoeynum, Periploca), in 

 merous Orchids (Ophrys, Spiranthes), in Fourcroya (nearly related to Agave), 

 d in several Bulrushes (Typha Shuttleworthii and latifolia), the four pollen-cells 

 3 arranged in one plane. In a few Willow-herbs (e.g. Epilohiv/m montanum and 

 rsutum) the four cells are joined, but so slightly that they are readily separated 

 pressure. 



Of much rarer occurrence than tetrads are pollinia. This name is given to the 

 raltant mass of poUen-ceUs, when the whole of the pollen produced from a single 

 ehesporium (i.e. the whole contents of a pollen-sac) remain joined together into 

 tissue. A pollinium may consist of 8, 12, 64, or even many hundreds of pollen- 

 lls. In the Mimoseee the pollinia, which are found serially arranged, are 

 iticular, egg-shaped, or globular in form; in the Asclepiads they are spatulate, 

 d consist of hundreds of poUen-cells. The pollen-masses of many Orchids are 

 lilt up of numerous little clusters of pollen-cells, and show a branching or lobing; 

 ch of these clusters or lobes consists of greater or smaller pollinia. The masses, 

 to which the pollinia of Orchids are aggregated, usually terminate in a stalk 

 lich is attached at its other end to a disc. This disc is so sticky that it readily 

 heres to any object coming in contact with it — a fact of significance in the 

 Qveyance of the pollinia from flower to flower by insects. 



The dimensions of pollen-grains are very various in difierent groups of plants, 

 lus, whilst in the Forget-me-not (Myosotis), Borage (Borago), Comfrey (Symphy- 

 m), and Boraginese generally, as also in Artocarpeae (e.g. Ficus), the pollen-grains 

 e very small, in Cannacese, Malvaceae, Cucurbitacese, and Nyctaginese, they are 

 latively large. The following table of diameters of poUen-grains shows the 

 .riation which exists in this respect:— 



'/OSOtlS 



thospermum affine, 

 cus pumila, ... 

 rinthe minor, 

 Mum vulgare, 

 lea mxcrophylla, 

 Mmnus cathartiea, 

 ringa vulgaris, 

 oe denticulata, 

 uxa angustifolia, 



Millimetres. 

 0-0025-0-0034 

 0'0042-0-0052 

 0-0045-0-0056 

 0-0050-0-0057 

 0-010 -0-014 

 0-018 -0-020 

 0-022 -0-032 

 0-024 -0-034 

 0-035 -0-050 

 0-055 -0-065 



Viola tricolor, 

 Convolvulus sepium, 

 Geranium Rohertianum, 

 Opuntia cynanchiaa, 

 Oxyhaphus nyctagineus, . 

 Morina Persica, ... 

 Cuaurbita Pepo, ... 

 Mirabilis longiflora, ... . 

 Cuaumis Melo, 

 Mirabilis Jalapa, 



Millimetres. 



0-062-0-071 

 0-076-0-084 

 0-085-0-094 

 0-15 -0-20 

 0-18 -0-22 

 0-19 -0-24 

 0-20 -0-23 

 0-20 -0-24 

 0-20 -0-24 

 0'22 -0-25 



The pollen-grains of the Marvel of Peru (Mirabilis Jalapa) are consequently 



VOL. II. 



