THE MEGASPORANGIUM AND SEED 233 



The typical condition, of course, for both monocotyledons and 

 dicotyledons so far as the coat or coats of the seed are concerned is 

 that shown in Fig. 173, since only rarely does the wall of the ovary 

 participate in the formation of the protective envelope. 



Certain other varieties are presented by the organization of the 

 seeds of angiosperms. For. example, the endosperm or food sub- 

 stance may be absent altogether, a condition illustrated by the 

 legumes and the Compositae among the dicotyledons and by the 

 Orchidaceae among the monocotyledons. Again, the nucellar or 

 megasporangial substance, usually absorbed as the development 

 of the endosperm and embryo proceeds, sometimes persists and is 

 then known as perisperm. Another variation which may present 

 itself is the development of a supernumerary integument, often 

 brightly colored, after the seed has been fertilized. This subsidiary 

 coat is. known as the arillus and is frequently found in families not 

 nearly related systematically. 



The organization of the seed in the pine is presented in Fig. 174. 

 for comparison with the angiospermous conditions illustrated in 

 Fig. 173. The integument is clearly distinguished as a hard in- 

 vestment surrounding the abundant endosperm. The food sub- 

 stance in the seeds of the gymnosperms is derived directly from the 

 transformation of the gametophyte and is not a new structure, as 

 is the case with the endosperm of angiospermous seeds. Within 

 the substance of the endosperm lies the embryo, with its narrower 

 end toward the micropylar region of the integument. The more 

 slender portion of the embryo or young sporophyte is the primary 

 root, which is capped with more or less lax tissues. These are the 

 remains of the suspensors which in the developing seminal organ 

 forced the young embryo down into the midst of the endosperm, 

 The larger end of the embryonic sporophyte owes its breadth to the 

 presence of numerous cotyledons or seed leaves, and these dis- 

 tinguish the embryo of the pine from those of the angiosperms. 



