1 196 A TEXTBOOK OF THEORETICAL BOTANY 



part, either to the median or to a lateral bundle of the perianth member, 

 according to its relative position. In both the latter cases the separation of 

 the two bundles upwards produces two bundles normally orientated, with 



the xylems inwards. 



The stamen is normally a one-trace organ, as we have previously men- 

 tioned (p. 1 1 19) but three-trace stamens occur in some of the lower families 

 of Dicotyledons and in a few these traces show a slight degree of branching 

 and anastomosis, which, of course, is greatly extended in the case of petaloid 

 stamens. Examples in normal stamens are: Talaiima, Himantandra, 

 Degeneria and several genera of Winteraceae, e.g. Belliolum ; all included in 

 the Ranales, in a broad sense. As these are all types with a distinctly 

 primitive floral organization, it looks as if the one-trace condition in higher 

 groups is secondary. (See Fig. 1165.) 



A peculiar anomaly has been noted in Parnassia, the stamen bundle 

 being diploxylic, that is having both centrifugal and centripetal xylem, the 

 latter often forming an arc enclosing some parenchyma cells between it and 

 the protoxylem. There are also indications of several phloem groups around 

 the bundle. This is interpreted by Arber as indicating that the fertile 

 stamens are reduced from stamen fascicles, which, according to this author, 

 are still represented in the flower by the fasciculated staminodes of the 

 inner whorl. Arber maintains that Parnassia should be included, on this 

 and other grounds, in the Hypericaceae. 



The primordial growth of all flower parts, it is claimed, follows the 

 general leaf-pattern and is comparable with the growth of bracts and other 

 leaves of limited development. Laminal growth is slight but marginal 

 growth is prolonged and, in the stamen filaments, growth in thickness is 

 due to a ventral meristem spread over the adaxial surface. 



The structure of the stamen in the Orchidaceae is unique in several 

 respects. There is usually only one functional stamen, adherent to the top 

 of the ovary to form a gynostemium. The pollen grains are cemented 

 together into a solid mass, called a poUinium, in each anther-lobe. The 

 cementing substance between the grains, known as viscin, is derived from 

 the breakdown of potentially sporogenous cells, and a downward prolonga- 

 tion of this degenerated sporogenous tissue forms a basal appendage of 

 viscin, called the caudicula. Below the stamen is a knob of tissue, called 

 the rostellum, supposed to be derived from the third lobe of the stigma. The 

 interior cells of this structure undergo the same degeneration into viscin, 

 after which its upper epidermis dissolves and the caudicula becomes 

 adherent to the sticky mass thus exposed, which now separates from the 

 surrounding cells and forms a cushion known as the retinaculum. It is by 

 means of the latter that the caudicula and pollinium become attached to the 

 head of a visiting insect, in the manner described in Chapter XXX. 



The motor-tissue in irritable stamens, such as those of Berheris and 

 Centanrea, consists of parenchyma of rounded outline, with rather large 

 intercellular spaces, resembling the cortex in the motor pulvini of Mimosa 

 piidica. The mode of operation is not definitely known but is probably 



