120 EMBRYOGENESIS IN PLANTS 



In the foregoing pages the writer has set out what may be regarded 

 as the classical account of the embryogeny of Isoetes and the discus- 

 sions associated with it. In a comprehensive experimental investigation 

 of several species of Isoetes, La Motte (1937) has shown, by holding 

 megaspores in agar in predetermined positions during fertilisation and 

 the development of the embryo, that when the mature female gameto- 

 phyte is fixed in a horizontal position, so that the neck of the arche- 

 gonium is also horizontal, i.e. at right-angles to the direction of the 

 force of gravity, the embryo emerges with the primary leaf pointing 

 upward and the primary root downward. Fig. 29e. When the gameto- 

 phyte is placed so that the archegonium neck is directed upward, i.e. 

 in the line of gravity, the embryo emerges in a horizontal position, Fig. 

 29f, the leaf later curving in an upward direction and the root down- 

 ward; and as the young sporophyte continues to grow it straightens 

 itself out so that all trace of its original horizontal position is lost. 

 Relative to the archegonial axis, however, the young embryos, both in 

 position and shape, are closely comparable in the two experiments. 

 When the prothallus is placed with the archegonial neck directed 

 downward, the embryo is again horizontal but inverted, and stands in 

 the same positional relationship to the archegonium as in the first two 

 experiments, Fig. 29g. Lastly, when megaspores were constantly 

 rotated in a horizontal klinostat, the position of the embryo relative to 

 the archegonium was the same as before. Relative to gravity, however, 

 the leaves of embryos in the rotated megaspores emerged at random. 

 These experiments show clearly that factors in the prothallus or 

 archegonium are of primary importance in determining the orientation 

 and initital stages in the embryonic development. Because of the 

 radial symmetry of the archegonium and prothallus, gravity also 

 becomes effective and serves as a stimulus towards the vertical develop- 

 ment of the leaf-root axis, the response to this stimulus being very 

 pronounced. La Motte has ascertained that the period when the 

 zygote can respond to the gravitational stimulus probably corresponds 

 approximately to the period when it undergoes its first mitosis (between 

 24-36 hours after fertilisation). 



The growth of the embryo is rapid: at 48 hours the embryo is 

 spherical or ovoid and consists of about 12-20 cells; at 72-76 hours it 

 is elongated, is more developed on one side than the other, and the 

 ligule can be distinguished; during the fourth and fifth days the con- 

 formation of the foot, the first leaf, and the sunken ligule becomes 

 evident ; by the seventh day the leaf, in which chlorophyll has already 

 been formed, is bursting out of the prothallus and the conformation of 

 the root can be observed. 



From his experiments La Motte had ample materials for histological 



