CONCRESCENCE 



53 



leaves. In reality what happens here is that a number of free primordia 

 of leaves are formed, but these are soon raised up upon a common 

 circular base. We may consider this ring as made up of the basal parts 

 of the leaf-primordia which from the beginning, or ' congenitally,' are 

 united with one another. In speaking later of the formation of the 

 flower I shall revert to this point ; meanwhile a few examples illustrating 

 the existence of an actual concrescence may be given. 



In the first place it is to be noted that the earlier the concrescence has 

 taken place the less evidence of it is there in the mature condition. The 

 false septum of the fruit of the Cruciferae, for example, is composed of 

 two portions which have united with one another ; the junction of the 

 cells takes place so early however that this is not visible usually in the 

 mature condition. The same is the case in the union of many carpels, in 

 the corolla of Ceropegia and other plants. A temporary or permanent 

 concrescence by sutural union occurs between the leaves of the perianth in 

 many cases of ' valvate ' vernation *. Either the epidermal cells of adjacent 

 leaves which touch one another grow in tooth-like between one another 



/ // m E/ 



FlG. 22. Scheme of development of three hairs arising close together. / cells from which hairs arise, I, 2, 3 

 //free development ; ///partial concrescence ; IV complete concrescence. 



and then we can speak of a ' cell-suture,' or the toothing involves only the 

 ribs and prominences of the cuticle and then we speak of a ' cuticular 

 suture.' In the latter case the concrescence takes place only after the 

 formation of the cuticle and is therefore different from the former ; and 

 it may be afterwards dissolved by partial resorption, as in the staminal 

 tube of Lobelia, according to Reiche. 



The instances I have mentioned are indeed of biological interest, yet 

 they have not the importance which attaches to congenital concrescence 

 a condition involving, as I have stated, altogether different incidents. 



Let us take a sample case, illustrated in Fig. 22. The three adjacent 

 epidermal cells, i, 2, 3, represented in /, are about to develop into hairs. 

 They may do so independently as at //. In an allied plant they grow 

 from the beginning as a single cell-body. This may be designated 

 congenital concrescence of single cells, an expression which has primarily 



1 See Reiche, Uber nachtragliche Verbindungen frei angelegter Pflanzenorgane, in Flora, 1891, 

 p. 435; Raciborski, DieSchutzvorrichtungender Bliitenknospen, in Flora, Ixxxi (Erg.-Bd. 1895), p. 151. 



