46 



Robinson : Spines of Fououieria 



On the lower side of the petiole of the primary leaves, a woody 

 thickening is developed (Figs, ii and 12, zc) which increases in 

 firmness as the leaf matures. This arises from the cortex, which 



even in the bud is proliferated to form 

 a thickened area on the outer side of the 

 base of each leaf (Fig. i, c). As the 

 leaf emerges from the bud a conical 

 portion of the cortex on the lower side 

 of the petiole begins to lose its paren- 

 chymatous character. The cells lengthen, 

 their ends become pointed (Fig. 2, s) and 

 their walls thicken ; these changes are 

 ,, . , , , J accompanied by a diminution in their 



Fig. I. Section through bud. '^ •' 



a, young leaf ; ^, cortex, thick- lumina (FiG. 3, s). With phloroglucin 

 ened to form spine ; v, vascular and hydrochloric acid they take the 

 ^^^^^^- violet color characteristic of sclerenchy- 



matous cells. The conversion of the parenchyma into scleren- 

 chyma is incomplete in F. splendcns, and a slender cone shaped 

 mass of thin-walled cells extends 

 about two thirds of the distance 



from the base toward the tip 



of the newly formed spine. This 



is illustrated by the diagrams of s 



longitudinal and transverse sec- j.^^ ^ p^^^;^^ ^^ longitudinal section 



tions shown in Figs. 4, 6 and 7, through petiole, b, cortex ; t, separating 

 /. About the time when the layer ; 5, sclerenchyma. 



leaf-blade is cast off, the parenchyma-core disintegrates and a hol- 

 low spine remains. In Foiiqiiieria 

 Macdoiigalii, on the other hand, 

 almost the entire mass of the cor- 

 tex on the outer (lower) side of 

 the petiole is transformed into 

 sclerenchyma, making a solid 

 heavy spine (Figs. 5 and 8, s). 

 The growth of the leaf and 

 Fig. 3. Portion of transverse section the development of the spine is 

 roug pe 10 e. accompanied by the differentiation 



of a separatory layer of thin-walled, elongated cells (Figs. 2 and 



