PTERIDOPHYTES. 107 



this must be used cautiously on account of the great delicacy of the 

 tissues. 



If a plant with ripe archegonia is placed in a drop of water, with the 

 lower surface uppermost, and at the same time male plants arc put with 

 it, and the whole covered with a cover glass, the archegonia and antheridia 

 will open simultaneously ; and, if examined with the microscope, we shall 

 see the spermatozoids collect about the open archegonia, to which they 

 are attracted by the substance forced out when it opens. With a little 

 patience, one or more may be seen to enter the open neck through which 

 it forces itself, by a slow twisting movement, down to the egg cell. In 

 order to make the experiment successful, the plants should be allowed to 

 become a little dry, care being taken that no water is poured over them 

 for a day or two beforehand. 



The first divisions of the fertilized egg cell resemble those in the moss 

 embryo, except that the first wall is parallel with the archegonium axis, 

 instead of at right angles to it. Very soon, however, the embryo becomes 

 very different, four growing points being established instead of the single 

 one found in the moss embryo. The two growing points on the side of 

 the embryo nearest the archegonium neck grow faster than the others, one 

 of these outstripping the other, and soon becoming recognizable as the first 

 leaf of the embryo (Fig 67, A, L). The other (r) is peculiar, in having its 

 growing point covered by several layers of cells, cut off from its outer face, 

 a peculiarity which we shall find is characteristic of the roots of all the 

 higher plants, and, indeed, this is the first root of the young fern. Of the 

 other two growing points, the one next the leaf grows slowly, forming a blunt 

 cone (st.), and is the apex of the stem. The other (/) has no definite 

 form, and serves merely as an organ of absorpti&n, by means of which 

 nourishment is supplied to the embryo from the prothallium ; it is known 

 as the foot. 



Up to this point, all the cells of the embryo are much alike, and the 

 embryo, like that of the bryophytes, is completely surrounded by the 

 enlarged base of the archegonium (compare Fig. 67, A, with Fig. 55) ; but 

 before the embryo breaks through the overlying cells a differentiation of 

 the tissues begins. In the axis of each of the four divisions the cells 

 divide lengthwise so as to form a cylindrical mass of narrow cells, not 

 unlike those in the stem of a moss. Here, however, some of the cells 

 undergo a further change ; the walls thicken in places, and the cells lose 

 their contents, forming a peculiar conducting tissue (tracheary tissue), 

 found only in the two highest sub-kingdoms. The whole central cylinder 

 is called a " fibro- vascular bundle," an,d in its perfect form, at least, is found 

 in no plants below the ferns, which are also the first to develop true roots. 



