352 SCIENCE PROGRESS. 



and the tissue derived from it are their chief seat, 

 and next in importance is the secondary bast. When 

 they occur in the wood, it is generally in the medul- 

 lary rays. The pith also shows their presence. 

 Guignard makes nine types of distribution in the 

 stems of Cruciferae : — 



( i ) The pericycle alone, e.g., Lepidium sativum, etc. 



(2) The pericycle, and the primary and second- 



ary bast, e.g., Erysimum cheiranthoides. 



(3) The cortex chiefly, but also the pericycle, e.g., 



M oricandia hesperidiflora. 



(4) The cortex and the bast, under a thin and 



sclerenchymatous pericycle, e.g., Iberisamara, 

 etc. 



(5) The cortex, pericycle, and pith, e.g., Nastur- 



tium officinale, etc. 



(6) The cortex, pericycle, and secondary bast, e.g., 



Bunias orientalis, etc. 



(7) The cortex, pericycle, secondary bast, and 



pith, e.g., Raphanus sativus, etc. 



(8) The cortex and pericycle chiefly ; also the 



primary and secondary bast and pith, e.g., 

 Brassica nigra, etc. 



(9) The cortex, pericycle, primary and secondary 



bast, woody parenchyma and pith, e.g., Coch- 

 learia armoracea, etc. 

 The distribution is somewhat similar in the other families 

 referred to. In the Limnanthea± they lie mainly in the 

 lacunar cortex, but a few are to be found in the bast ; in 

 the Tropaiolacea; they generally form groups or nodules of 

 cells in the hypodermal layer of the cortex, a few being in 

 the bast. 



Leaves. — Species that contain many of the secreting cells 

 in the axis of the plant generally contain them in the 

 leaves also, and the relative proportion is often 

 greatest there. They occur throughout the meso- 

 phyll, but are usually most numerous towards the 

 lower surface. In some leaves they are localised in 

 the mesophyll and the pericycle ; in others, chiefly 



