THE POLLEN. 295 



side, which corresponds to the margin of the leaf and the line of 

 dehiscence. This appearance is presented by a large number of 

 full-grown anthers : but the partition usually disappears before the 

 anther opens, when each lobe becomes single-celled. The normal 

 anther is consequently considered as two-celled. In Menispermum 

 and Cocculus (Ord. Menispermacese)', however, the anther is 

 strongly four-lobed externally, and each lobe forms a distinct cell, 

 at maturity. Although the stamens originate a little later than the 

 petals, when these are present, yet they outgrow them at first, and 

 their formation is earlier completed (489). 



532. The Pollen, contained in the anther, which appears to the 

 naked eye like a mere powder, consists of grains of definite size 

 and shape, which are uniform in the same plant, but often very 

 different in different species or natural families. Although com- 

 monly spherical or oval, they are cylindrical in the Spiderwort 

 (Tradescantia), nearly square in Colutea, many-sided in the Tea- 

 sel, and triangular, with the angles dilated and rounded, in the 

 Evening Primrose (Fig. 419). The most remarkable shape is 

 that of Zostera (a marine aquatic plant), in which the grains con- 

 sist of long and slender threads, which, as they lie side by side in 

 the anther, resemble a skein of silk. Their surface, although more 

 frequently smooth and even, is banded or crested in many cases ; 

 it is reticulated in the Passion-flower, and studded with strong 

 points in Convolvulus purpureus (Fig. 417), or short bristles in the 

 Mallow Family and the Gourd. The color is usually yellow. 



533. The grains of pollen are single cells, formed usually in 

 fours, by the division of the living contents of mother cells first 

 into two, and these again into two parts, which, acquiring a layer 

 of cellulose, become four specialized cells, nearly in the manner 

 already described (31, 95). As the pollen completes its growth, 

 the walls of the mother cells are usually absorbed or obliterated, 

 when the grains lie loose in the cell. But sometimes the inclosing 

 cells persist, and collect the pollen-grains into coherent masses of 

 various consistence, as in the Milkweed Family (Fig. 422) and in 

 the Orchis Family (Ord. Orchidacere). Such pollen-masses are 

 sometimes called pollinia. The threads, like cobweb, that are 

 loosely mixed with the pollen of the Evening Primrose (Ord. Ona- 

 gracese), are vestiges of nearly obliterated mother cells. 



534. Not unfrequently the four grains developed in the same 

 cell cohere, more or less firmly, as in most Ericaceous plants ; or 



