200 PROCEEDINGS OF THE AMERICAN ACADEMY 



This brief sketch shows how the rhizogen with its two sets of hap- 

 teres develops. They do not increase iu complexity to any extent 

 during this period. But while this has been taking place there have 

 been important changes in the other organs of the plant. 



If we start at the tip of the blade of a specimen which has the first 

 set of hapteres well developed, we shall find it to possess a well defined 

 limiting layer on each surface, and within this, on both sides, a cortical 

 layer of a single series of cells, and in the centre the medulla, repre- 

 sented by a few scattered tubes. As we approach the base, we find 

 the limiting layers more and more sharply defined. Their cells are 

 smaller and more elongated radially, and give evidence of greater 

 activity. Within these the cortical layers are more developed, and 

 are to be distinguished mto an inner and an outer layer. The outer 

 layer consists of smaller pai'enchymatous cells, while the inner con- 

 tains laiger elongated cells. We have, too, at this point a distinct 

 medulla, m which the long tubular cells form a conspicuous element. 

 Surrounding them are elongated tube-like cells of an appearance in- 

 termediate between that of the original tubes and the elongated inner 

 cells of the inner cortex. Their walls are thin, while the walls of the 

 original tubes are now noticeably thickened, and they are in general 

 destitute of the dense contents of the latter. They have attached to 

 them and entwining about them numbers of hyplial filaments, which 

 usually have a lateral or oblique direction of growth. 



If one examines the mnermost rows of cells of the inner cortex in 

 transverse, or better in longitudinal section, he will see whence these 

 hyphffi arise. As the cells of the inner cortex elongate, the middle 

 layer of the common wall becomes swollen, and the cells become 

 finally separated from one another, except at a few points on each 

 face. As the layer continues to swell and to become indistinct, the 

 connections at these points become drawn out into short tubes ; and 

 since the process continues, these may become quite long at times. 

 The tubes or hyphie may be known as connecting hypha3 (Verbiud- 

 ungshyphen). In the other LaminariecB, as far as investigated, they 

 are said to arise only at points on the walls where there are pits, and 

 this seems to be true of the specimens examined ; at any rate, iu all 

 places where the point of origin could be plainly seen, this was the 

 case. In the region where the cells begin to separate, the cell walls 

 proper are thin, and the pits are not conspicuous. But if the section 

 be stained with a dilute solution of Boehmer's ha^matoxylin the 

 appearances shown in Figure 23 are brought out. In this figure it 

 is seen that at the place where the two cell walls remain in contact 



