42 STRUCTURAL BOTANY. 



in Orchis. Inequality in length is definitely marked in two 

 cases, as tetradynamous (<reTpa$, four, i^vajju.c, power) when the sta- 

 mens are six, whereof four are longer than the other two,' as in 

 all the Crucifers; didynamous, where the stamens are four, two 

 of them longer than the other two, as in all the Labiates (104, 106). 

 120. Cohesion is as frequent with stamens as with petals. 

 They are monadelphous (<x<jX<p6$r, a brother) when thyy are all 

 united, as in Mallow, into one set or bi'otherhood by the fila- 

 ments; diadelphous in two sets, whether equal or unequal, asjn 

 Pea, Squirrel-corn ; polyadelphous, many sets, as in St. Johns- 

 wort ; and syngenesious, when they are united by their anthers, 

 as in the Composite. Finally, the absence of the stamens alto- 

 gether, whether by abortion, as in the 9 flowers of Veratrum, 

 or by suppression, as in Oak, occurs in various modes, render- 

 ing the plant inono3cious ( 8 ), dioecious ($ $ ), or polygamous 

 (5 $ ? ), as already explained ( 67). 



Pollen grains. 107, Finns larico. 108, Basellarubra. 109, Ranunculus repens. 110, Scolymus grandi- 

 tiorus. Ill, Passiflora incarnata. 



121. The pollen is in appearance a small, yellow dust, con- 

 tained in th-e cells of the anther. When viewed with the mi- 

 croscope it appears as grains of various forms, usually spheroidal 

 or oval, sometimes triangular or polyhedral, but always of the 

 same form and appearance in the same species. Externally they 

 are curiously, and often eLegantly figured with stripes, bands, 

 dots, checks, etc. Each grain of pollen is a membranous cell or 

 sack containing a fluid. Its coat is double the outer is more 

 thick and firm, exhibiting one or more breaks where the inner 



