318 PKACTICAL BOTANY 



a close physiological connection between the prothallus 

 and the Fern-plant. 



c. A first leaf, or cotyledon, with an elongated 

 petiole, and bifurcating, expanded, upper part: this 

 usually grows upwards through the depression at the 

 apex of the prothallus. 



d. Between the base of the cotyledon and the foot is 

 the apex of the stem, which continues its growth, 

 and produces new leaves. 



Having thus gained a knowledge of the position of the several 

 parts relatively to one another, and to the prothallus, in the case 

 of a young Fern-plant of considerable size, younger plants may 

 successively be taken, and by a comparison of these the mode 

 of development of the young embryo, or Fern-plant, may be 

 traced. In order to make preparations so as to show the several 

 parts of the embryo, sections should be made either from fresh 

 material, or better from material prepared with picric acid, and 

 hardened in alcohol, as above directed for the prothalli. The 

 direction of section should be parallel to the organic axis of the 

 prothallus, and perpendicular to the flattened surface : in such 

 sections, including embryos of suitable age, the stem, cotyledon, 

 root, and foot may all be observed ; further, the origin of these 

 several parts from definite segments of the fertilized ovum may 

 be traced. For details as to the sequence of cell-divisions in the 

 first stages of development of the embryo, reference should be 

 made to text-books. 



