IO2 



A TEXT-BOOK OF BOTANY. 



Angiosperms but also in the anatomical structure of the stem, 

 which is without large conducting vessels. In order to understand 

 the relation of the Gymnosperms to the Pteridophytes on the one 

 hand and to the Angiosperms on the other, it will be necessary to 

 consider briefly the life history of a representative group, such as 

 the Coniferae. 



General Characters. The seed consists essentially of three 

 parts, namely, a woody or leathery seed-coat, a nutritive layer 

 rich in oil known as the endosperm, and a straight embryo. The 

 latter is a more or less differentiated plantlet, consisting of a stem 



spm 



FIG. 63. The female gametophyte of a Selaginella; prothallus (pr) projecting through 

 the ruptured wall (spm) of the megaspore; ar, sterile archegonium; embi, emb2, two embryos 

 embedded in the tissue of the prothallus; et, et, suspensors. After Pfeffer. 



with a varying number of cotyledons or first leaves (2 to 16), 

 and a small root which is attached to a suspensor, as is the embryo 

 in Selaginella (Fig. 63). When the embryo begins its develop- 

 ment into the plant it uses up the nourishment with which it is 

 surrounded in the endosperm, and as it increases in size the seed- 

 coat is split. The root then protrudes and the cotyledons, to some 

 of which the seed-coat is still attached, are carried upward by the 

 stem through the surface of the soil, when the seed-coat is cast 

 off and the plant begins an independent existence. The first root 

 is the primary or tap root, and from this are sent out numerous 

 branches known as secondary roots, constituting a well-developed 



