THE ANATOMY OF THE STEM OF THE PAPAVERACE^E. 605 



7. Peculiarities of the Cortex of Romneya trichocalyx. 



"Arms" of rather thick-walled cells span the gap between the pericycle and 

 epidermis. These appear to be primarily for support, and may be compared with the 

 collenchymatous pads of Esohschohia calif omica. 



Summary. 



1. The stem structure throughout the Papaveracese is very uniform, the chief 

 characteristics being 



(a) A subepidermal band of chlorophyll-bearing tissue. 



(b) A sclerotic pericycle (except in Argemone mexicana and Romneya 



trichocalyx). 



(c) A massive pith, often fistular, e.g. Hypecoum procumbens. 



Minor differences occur which are constant for individual genera and thus 

 useful for classification purposes, e.g. 



(a) The amount of cortical tissue ; e.g. two layers in Chelidonium majus, 



the outer being a colourless exodermis, the inner only being green, or 

 several layers, as in Papaver orientate and P. somniferum. 



(b) The distribution of chlorophyll. All the cortical cells may contain 



chlorophyll, e.g. Papaver Rhosas, or only some of them, e.g. P. orientale, 

 Corydalis racemosa, Adlumia cirrhosa, Romneya trichocalyx, and 

 Dicentra formosa. 



(c) The intensity of sclerosis in the pericycle, which may vary considerably. 



In Romneya trichocalyx and Argemone mexicana the pericycle is not 

 sclerotic, in Papaver pilosum and P. Rhoeas it is entirely so, while it 

 it is only feebly sclerotic in Eschscholzia califomica, Corydalis race- 

 mosa, Papaver somniferum, and P. orientate. 



(d) The thickness of the pericycle. In the fascicular regions in Meconopsis 



the pericycle is usually five or six layers deep, while in Hypecoum and 

 Eschscholzia it consists of only one layer. In the interfascicular 

 regions it usually merges gradually into the non-sclerotic medullary 

 rays, but in Adlumia cirrhosa there is an abrupt change from 

 sclerotic to non-sclerotic tissue in this region. 



(e) The degree of differentiation of the exodermis. The exodermis (of Leger) 



is well marked in some genera, e.g. Chelidonium, in others, e.g. 



Papaver Rhosas, scarcely at all. 



(/) The distribution of the vascular bundles. These are numerous, and 



arranged in a single compact circle in Argemone mexicana, they are 



few and far apart in Hypecoum procumbens, numerous and arranged 



in a monocotyledonous manner in P. orientale and P. somniferum. 

 TRANS. ROY. SOC. EDIN., VOL. LI, PART III (NO. 18). 88 



