220 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



position of the embryo of Marsilia *. ' The position of the first division- 

 wall of the pro-embryo of Marsilia is so far definite and independent of 

 external relationship that it always falls in the plane of the axis of the 

 archegonium. It is however capable of torsion about the latter, and in 

 the event of the axis of the archegonium deviating from the vertical 

 it assumes such a position that the embryo is laid with an upper half 

 directed to the zenith, the epibasal half, forming the stem, and an under 

 half, the hypobasal half, forming foot and root.' The macrospores 

 germinate in a position approaching the horizontal (Fig. m); the 

 result of the disposition just indicated is that the stem-apex, j, is in every 

 case turned upwards, whilst the root is turned downwards, and in this 

 way all curved growth is avoided which must take place were the dis- 

 position otherwise. The macrospores are liable to torsion ; in the homo- 

 sporous ferns on the other hand the prothallus is fixed to the ground. 



FlG. in. Marsilia. Half diagrammatic longitudinal section through a macrospore after germination. To the 

 right the macrospore; to the left the prothallus upon which one embryo is developed. The neck of the arche- 

 gonium is turned obliquely downwards. The pluricellular embryo shows cotyledon c, stem-apex j, foot f, and 

 root. The first division-wall in the fertilized egg is still visible and is indicated by //. The embryo at the stage 

 represented would of course exhibit a construction of many cells, but none of these are shown excepting those of the 

 stem-apex. 



The reciprocal position of parts is also in the embryo of Marsilia not 

 displaceable ; the foot,/, is always as its function requires turned towards 

 the spore in which the reserve-material is stored, the cotyledon and the 

 root are always opposite to it. If the axis of the archegonium be turned 

 so as to be directed vertically upwards or downwards, a condition which 

 really never occurs in nature, gravity cannot of course exercise any 



1 Leitgeb, Zur Embryologie der Fame, in Sitzungsb. d. Wiener Akademie, 1878 ; Id. Studien liber 

 Entwicklung der Fame, ibid. 1879. From what is said in the text it is evident that external stimuli 

 play no part or only a subordinate one in the differentiation of organs in the fertilized egg of the 

 Pteridophyta ; the lie of the egg within the female organ is the important matter. The same may 

 be said of the Spermaphyta ; the root of the embryo always arises on the attached side of the egg no 

 matter what position this occupies in relation to the horizontal. This is evidently connected with 

 the enclosed position occupied by the egg ; in the free eggs of Fucaceae external agencies influence 

 the polar differentiation as will be pointed out later. 



