OF ARTS AND SCIENCES. 199 



and the swelling called the rhizogen results. It is produced by the 

 great increase in the size of the cells, not by any large increase in the 

 number of cells, as Barber* has already described for the same organ 

 in Saccorhiza bulbosa. 



Not only do the inner cells of the rhizogen enlarge, but the walls 

 thicken and separate from one another, except at certain points on 

 each face, wiiere they still remain united. The space between the 

 separated walls is filled for some time with a mucilaginous modifica- 

 tion of the outer layers of the cell wall (cellulose) ; but after a while 

 it is impossible to detect anything of this kind, and the substance 

 probably becomes very watery indeed. However that may be, the 

 spaces between the walls finally become filled with small cells in lines 

 and rows, the origin of which is not readily determined. They ap- 

 pear to bud out from the larger cells in the form of small hyphae, 

 which, however, become so crowded that they seem to form a compact 

 tissue almost at once, and to lose all trace of their hyphal structure. 

 (Cf. Fig. 21,^..) 



The limiting layer has continued growing, and at certain places 

 along the equator of the rhizogen, in company with the underlying 

 cortical cells, it takes on a much greater activity than it does else- 

 where. The increase at these portions is in the radial direction, and 

 is brought about both by the tangential division of the cells of the 

 limiting layer and by the increase of the cortical cells in the radial 

 diameter. As a result several protuberances are formed in the zone 

 of the equator destined to push out into hapteres. The cortical cells 

 increase to from six to eight times their original radial diameter, and, 

 new cortical cells being supplied by the activity of the limiting layer 

 at the distal end of each protuberance, the haptere begins to elongate. 

 The limiting layer is more active at the summit of a haptere than 

 along its sides ; in fact, very few new cells are formed in the latter 

 region at first. Later, however, when it has finished its growth in 

 length, these conditions are reversed, and its growth is then in thick- 

 ness. When the haptere has reached the substratum in its outward 

 and downward growth, the cells of the limiting layer along the surface 

 in contact with the substratum grow out into rhizoids for attachment 

 like those mentioned in the case of the primitive disk. 



The second set of hapteres arises after the first set is fully formed. 

 They develop in exactly the fashion that the members of the first 

 set did. 



* Annals of Botany, Vol III p. 55. 



