SIMPLIFIED LEAVES 19 



COTYLEDONS. 

 One characteristic of an ordinary Dicotyledon, the Bean 

 for example (fig. '2), is that the first two leaves produced 

 by the embryo are inserted opposite to each other on the 

 primary axis, and in form they differ from the subsequently 

 produced foliage-leaves. These two leaves are known as 

 the cotyledons. [Frequently the cotyledons are termed 

 seed-leaves, because they are found inside the seed. But 

 this expression seed-leaves is a bad one, because it seems 

 to suggest that the seed bears cotyledons, whereas it is to 

 the stem of the embryo inside the seed that these leaves 

 belong.] Cotyledons are simple leaves, usually with margins 

 devoid of any incisions ; only very rarely do they possess 

 stipules or definite sheaths, though occasionally they have 

 distinct stalks. When the 

 seed germinates the cotyle- sc f j^ 



dons may remain hidden in \ i , l.,||,_ |/ 



the soil, in which case they ^8*f?^?-..-^^^^^»^^ 



are not green in colour — e.g. ^^:'-'-:'-'-''^-C-:------^^^^^^ 



Bean ; but in other cases ^y^^sjr^':-::-'y^'-''^-^l-ii^^^^ 

 the cotyledons emerge (fig. 4) ''''"^Jj^i^^V.y-y.?-:^^^;^^^ 

 from the soil and become f ^^nnii^^^BB**^^ 

 green — e.g. Beech, Sycamore, '- 



Mustard ^^ 



iViUSLaiu. , T^. ^ 1 Fig. 28.- Vertical section through a 



Opposed to the Dicotyle- grain of viTieat. 



dons are Monocotyledons, 



whose embryos possess only one cotyledon each. That is to 

 say, the first leaf formed by the embryo is attached to the 

 axis at a lower level than any other leaf. We may select 

 the grain of the Wheat, for the purpose of examining a 

 Monocotyledonous embryo. 



Structure of a grain of Wheat (fig. 28). — As will be 

 shown later, the grain of wheat is not a seed, but is a fruit 

 with a single seed which occupies the whole of its cavity. 

 The grain is smooth and convex on the one side, and 

 deeply furrowed down the middle of the other side. Its 

 main mass is formed by a substance termed the endosperm 

 (e). The embryo is a small body placed at the bottom 

 corner of this endosperm. Looking at the convex surface 

 of the grain from the outside, there is a pale patch which 

 indicates the position of the embryo. The latter is rather 



