THE STAMENS. 



121 



fertilizing material ; but in other instances, as in the Bee Orchis (Fig. 2uO), it is pro- 

 bable that insects and birds are the means of con- 

 veying the pollen to the pistil. 



Before we describe the influence of the pollen, it 

 is needful to refer to the anatomical characters of 

 that substance. The pollen appears to the naked eye, 

 or with a lens of low power, to consist of a number 

 of particles or granules, of various sizes and figures, 

 which are technically termed cells. The more com- 

 mon figure (as is the case in all cells which lie loose), 

 is spherical, or ovoid ; but the most diverse forms have 

 been noticed. Thus they are square in the Bladder 

 (Senna), and tri- 

 angular in the 

 evening primrose 

 (Fig. 233). In 

 various com- 

 pound flowers 

 they are many- 

 sided ; in other 

 plants they are 

 twisted; and in 

 Dill they are cy- 

 lindrical. Such, 

 however, are ex- 

 ceptional cases ; 

 and whether they 



Fig. 230.-An Orchis, with its gy- may be attributed 

 nandrous flower. 



to pressure as in 



the cells of cellular tissue, is not known. 



When examined with high magnifying powers, as with the eighth of an inch ob- 

 ject glass, they are found not to be simple cells, but cells having a cell- wall divisible 

 into two or three layers, and inclosing a turbid-looking fluid, termed fovilla. The ex- 

 ternal layer of the cell-wall is usually itself composed of cells, and is called the extine ; 

 whilst the inner one is of greater delicacy and extensibility, and known as the inline. 

 In some instances, as in the Yew, there is a third membrane between these two, and 

 named the exintine, whilst in the Evening Primrose a fourth has been described as the 

 intexine. It is probable that all pollen cells have the two former ; but it is not indubi- 

 table at present that the two latter are at all commonly found. 



Thefovilla usually consists of two portions, which are in constant motion, as may 

 be seen in the garden plant, Clarlcia pulchella, one of which is larger and more oblong 

 than the other ; and as it differs from all other vegetable structures, it is presumed to 

 be the fructifying substance. 



Such is the structure of the pollen before it is applied to the stigma ; but after it 

 has commenced its fructifying function it exhibits characters unseen before. Thus, 

 immediately it has fallen upon the soft viscid tissue of the pistil, it begins to emit one 

 or more minute processes, which traverse the length of the pistil, and are called pollen 

 tubes (Figs. 232.233). These tubes terminate in the placenta, and thus constitute a medium 



Fig. 231. Exhibiting adnate stamens 

 and a pistil elevated much above the 

 stamens. 



