24 FAMILIAR TREES 



pollinated, the stigma may by this curvature of the 

 style be brought into contact with any remaining pollen 

 of its own flower. 



The blossoms are followed by silky green pods 

 which turn black as they ripen, and then burst 

 elastically by the contraction on drying of certain 

 oblique groups of cells in each valve. Thus the black 

 seeds are scattered to some little distance. 



All the green parts of the plant are poisonous, con- 

 taining an irritant emetic principle known as cytisin ; 

 but this appears to exist in a peculiarly concentrated 

 form in the seed. In its germination this seed re- 

 sembles a bean rather than a pea, since the cotyledons 

 rise above ground, become green and act as the two 

 first foliage-leaves, whereas those of the pea remain as 

 mere storehouses of nutriment within the seed. These 

 seed-leaves are stalkless, elliptical, and fleshy, in no 

 way resembling the subsequently-produced foliage, 

 and a slight want of symmetry in the seed renders the 

 two halves of each cotyledon unequal. 



Of considerable interest is Cytisus Adami Poir., 

 often known in gardens as G. purpuras'cens, or Purple 

 Laburnum, which bears three kinds of blossoms, 

 yellow, purple, and an intermediate brick-red, on one 

 tree, or even in one cluster. It originated in Paris in 

 1828, when M. Jean Louis Adam inserted a bud of the 

 Weeping Purple Broom, C. purpu'reus Scop., into a 

 Laburnum stock. Apparent^ a " graft-hybrid " was 

 formed, the two species becoming intimately united in 

 their growing-tissue or cambium. 



