252 



ORGANOGRAPHY. 



The dehiscence is produced, partly by the development and 

 growth of the pollen in the anthers pressing upon the walls and 

 causing an absorption of their tissue ; and partly by the special 

 action of the fibrous tissue Avhich forms the lining of the anther. 

 Thus Ave have already seen that the endothecium is composed 

 of fibrous cells and that it is altogether wanting at the su- 

 tures, and that the exothecium is also commonly very thin at 

 those points. The manner in which the fibrous layer acts is 

 thus stated by Jussieu : — "The small, very elastic hygrome- 

 tric threads or bands constituting this layer, stretch, contract, 

 lengthen, and bend in different ways, according as the anther is 

 dry or moist ; and these variations are influenced, on the one 

 hand by the development of the anther, the juices of which, at 

 first abundant, are absorbed and eA'ai)orated by degi-ees ; on the 

 other hand, by the variable state of the atmosphere. The tissue 

 which forms the wall of the anther, thus subjected to a series of 

 forces acting in diiferent directions, naturally breaks where it 

 ofiers but little resistance, that is, at the line or the point where 

 the fibrous layer is interrupted ; and it is thus that the loculus 

 splits and communicates Avith the exterior, so as to allow the 

 free egress of the pollen enclosed in the cavity, an egress, which 

 the continual contractions of the elastic tissue favour, and then 

 complete." 



The dehiscence may take place in four different ways, which 

 are called, 1. Longitudinal, 2. Transverse, 3. Porous, 4. Val- 

 vular. 



1. Longitudinal dehiscence. This the usual 

 mode, consists in the opening of each anther 

 lobe from the base to the apex in a longitudinal 

 direction along the line of suture, as in Wall- 

 flower (Jig. 423) and Buttercup. 



2. Transverse. — This kind of dehiscence mostly 

 occurs in unilocular anthers, as those of Alche- 

 viilla {fig. 504), Lemna, Lavandula, &c. It sig- 

 nifies that the splitting open of the anther occurs 

 in a transverse or horizontal direction, i.e. from 

 the connective to the side. It sometimes hap- 

 pens that by the enlargement of the connective 

 the loculus of a one-celled anther is placed hori- 

 zontally instead of vertically, in which case tiie 

 dehiscence when it takes place in the line of 

 the suture would be apparently transverse, al- 

 though really longitudinal. An exami>le of this 

 kincfof dehiscence is attbrded liy the Mallow 

 {Malva),i\m\ other allied plants {fig. 523). 



3. Puruus or Apical.— Thin is a mere modifi- 

 cation of longitudinal dehiscence. It is formed by the splitting 

 down of the anther lobes being arrested at an early period 



Fig. 523. 



F!g. r>2^. Stamen 

 of Die MuMow 

 (Mdlid). tlie an- 

 ther otwhiclilius 

 an apimrently 

 transverse de- 

 hiscence. 



