204 



THIRD GROUP. VASCULAR CRYPTOGAMS. 



The mode in which the oospore developes into the embryo is essentially the same 

 in all the Vascular Cryptogams that have been carefully observed. The first point 

 to notice is that the primary stem, the primary root and one or two leaves, here as in 

 the Phanerogams called cotyledons, are produced independently of each other from 

 the embryo, which is at first round or ovoid and unsegmented. A rather large 

 portion of the embryo is also devoted to forming an organ of suction, the foot, 

 which conveys nutrient substances from the prothallium to the embryo. The organs 

 which subsequently appear, the leaves, the roots, &c., are as usual produced at the 

 apex of the stem, but the members of the plant which are first developed on the 

 embryo originate, as has been said, in a different manner. As regards the orientation 

 of the different organs, it has been shown that the neck of the archegonium is turned 

 downwards towards the ground. The rudiments of the apex of the stem and of the 

 foot lie on the side of the embryo which is towards the under side of the prothallium, 



FIG. 152. Archegonium of Adiantum Capilfas-Veneris, magn. 800 times. A, B, C, E in optical longitudinal section, 

 D in optical transverse section ; A, B, C before, E after fertilisation has taken place ; h neck of the archegonium', 

 st canal-cells converted into mucilage, j in B the ventral canal-cell, e the oosphere, e in E the two-celled embryo. After 

 lying nine days in glycerine. 



and are therefore turned upwards ; the cotyledon and the rudiment of the root on 

 the side towards the neck of the archegonium, and are therefore directed down- 

 wards. The view which has been repeatedly expressed, that external forces, that of 

 gravitation especially, affect the position of the organs in the embryo, has not been 

 found to be correct ; their formation is determined only by the position of the embryo 

 in the prothallium and archegonium, and is wholly independent of gravitation *. 



The steps by which the oospore is changed into a mass of cellular tissue 



1 Leitgeb, Studien u. Entwicklung d. Fame (Sitzungsber. d. KK. Akad. d. Wiss., Bd. LXXX). 

 The fact alleged in the text was demonstrated by Leitgeb in a simple manner ; he showed that the 

 organs are formed in the embryo in the ordinary position in archegonia on the upper side of pro- 

 thallia, on which the light was thrown from below. 



