FLOWERING PLANTS OF TUB GREEN LANES. 257 



(C-eniaurca, Fig. 195, JP) and the Shallot onion. And it 

 is found that these shapes are, in some instances, 

 characteristic of different species in the same genus. 

 Thus their investigation may help in approximating 

 the values of specific differences one of the most 

 vexed questions in modern botany. The sphas- 

 rapkides are groups or conglomerations of minute 



Fig. 195. 



p, Crystal Prism from ovary -coat of Centaurea nigra ; q, different 

 forms, one in its cell, from the same part of Centaurea scabmsa; r, 

 Crystalline Cross, and three single Crystals, from bulb-scale of Shallot. 

 All highly magnified. 



crystalline granules, or angular crystals, frequently 

 smoothish, and often granular or star-like on the 

 surface ; and these are generally contained in a 

 distinct cell. Their usual diameter is about the 

 thousandth part of an inch ; but in our British 

 plants their size is often smaller than this. They 

 reach their largest size among plants of the Cactus 

 tribe. The common Nettle, and Marine Bladder 

 tampion a plant resembling, in flower, the 



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