POLLEN. 



97 



uiotner-cells, united together in fours, and in this condition they leave the aijther- 

 cavities. These Httle pollen-aggregates are termed tetrads. Examples of such 

 plants are: — the Ericaceae {Erica, Calluna, Menziesia, Andromeda), the Bearherry 

 (Arctostaphylos), the Strawberry Tree (Arbutus), the Alpine Rose (Rhododendron), 

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

 Epacris and Leucopogon; many Winter-greens (Pyrolaceae); a few Sedges (Juncus 

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

 just enumerated the tetrads correspond to the four pollen-cells in the corners of the 

 mother-cell (cf. fig. 219 ^); but in the Apocynaceae (Apocynum, Periploca), in 

 numerous Orchids (Ophrys, Spiranthes), in Fourcroya (nearly related tx) Agave), 

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

 are arranged in one plane. In a few Willow-herbs (e.g. Epilubium inontanum and 

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

 on pressure. 



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

 resultant mass of pollen-cells, when the whole of the pollen produced from a single 

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

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

 cells. In the Mimosege the pollinia, which are found serially arranged, are 

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

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

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

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

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

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

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

 conveyance of the pollinia from flower to flower by insects. 



The dimensions of pollen-grains are very various in different groups of plants. 

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

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

 are very small, in Cannaceae, Malvaceae, Cucurbitaceae, and Nyctagine^B, they are 

 relatively large. The following table of diameters of pollen-grains shows the 

 variation which exists in this respect: — 



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

 Vol. II. 87 



