244 ORGANOGRAPHY. 



succession ; and in the latter case the dehiscence may either 

 commence with the outer stamens, or with the inner. Thus 

 in Helleborus, the outer stamens open their anthers first, and 

 those in the centre last ; while in Glaucmm the inner stamens 

 open first, and those of the circumference last. In the common 

 Eue {Buta graveolens) again, where there are two whorls of 

 stamens, the outer stamens incline towards the pistil first and 

 discharge their pollen, and then return to their former posi- 

 tion ; the inner stamens then incline and discharge tiieir pollen 

 and return in a similar manner. Sometimes, as in Parnassia 

 palustris, each stamen curves in succession towards the pistil, 

 and the anthers then open and emit their pollen. Usually the 

 outer stamens are those which dehisce first. 



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 cells which form the lining of the anther ; 

 and it takes place commonly at the sutures, because at these 

 parts, as we have already seen, the endothecium is altogether 

 wanting, and the exothecium is also commonly very thin, so that 

 the sutures are the weakest points of the anther-walls. 



The dehiscence may take place in four diflTerent ways, which 

 are called, 1. Longitudinal, 2. Transverse, Z. Porous, 4. Valvular. 



1. Longitudinal. — This, the usual mode of de- 

 Fig. 521. hiscence, 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 the 

 Wallflower {fig. 421), and Tulip {fig. 508). 



2. Transverse. — This kind of dehiscence mostly 

 occurs in unilocular anthers, as those of Alche- 

 7mlla {fig. 502), 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 the 

 dehiscence when it takes place in the line of 

 the suture would be apparently transverse, al- 



521 stMTnen ^^^^ugh really longitudinal. An example of this 

 of the Maiufw kind of dehiscence is afforded by the Mallow 

 tS^of whfdi (^^^^^^ (fi9- ^21), and other plants belonging to 

 hasan°apparent- the natural order Malvaceae. In practical botany 

 ly transverse de- g^^g]^ anthers, like the former, are said to dehisce 

 hiscence. , ' 



transversely. 



3. Porous or Apical. — This is a mere modification of longi- 

 tudinal dehiscence. It is formed by the splitting down of the 

 anther lobes being arrested at an early period so as only to pro- 



