on the green parts of Hibiscus vitifolius Linn. 197 



in which are a few stomata (figs. 14, 15 and 16). Below this is a 

 layer, usually only one cell deep, but sometimes two or more, of 

 thin-walled parenchyma containing chlorophyll and crystals of 

 calcium oxalate. Within this is collenchyma, which also contains 

 chlorophyll. 



(1) The formation of the outgrowth originates in the epidermis 

 and in the subepidermal parenchyma. The latter cells first divide 

 by periclinal walls and elongate radially, so that the epidermis 

 with its stomata and hairs is lifted up (fig. 14). It is noteworthy 

 that on the outgrowths of the stem stomata are very few ; probably 

 this is to be correlated with the small number on the normal 

 epidermis of the stem as compared with that of the leaf. The 

 walls of the parenchymatous cells formed by these divisions sub- 

 sequently become much thickened (by cellulose) and strongly 

 pitted. This green tissue may be many (5, 6 or more) cells deep 

 or it may be almost entirely absent. As in the leaf it forms a sort 

 of cushion on which the upper part of the outgrowth is placed. 

 The overlying epidermal cells also divide by periclinal walls and 

 elongate radially, at the same time becoming considerably distended 

 and very thin-walled. All these cells are colourless and contain 

 numerous leucoplasts and usually also oil drops. In this condition 

 all the cell walls consist of cellulose, and in sections treated with 

 gentian violet and eosin they acquire a uniform pink colour. 



(2) At a later stage the outgrowths begin to shew signs of 

 degeneration; the thick-walled lower cells begin to lose their 

 colour because the chloroplasts become smaller and paler (fig. 15). 

 The protoplasmic contents also become somewhat reduced, but the 

 cells of the outgrowth still retain their turgidity, their protoplasm 

 and their large nuclei. 



(3) In the last stage (fig. 16) the outgrowth becomes cut off 

 by the formation of cork, and this before any cork is developed in 

 the adjoining parts of the stem. The cork arises in the lowermost 

 of the daughter cells derived from the original epidermal cells, 

 i.e. from the lowermost of the colourless cells. In these cells a 

 new set of periclinal walls arises and may be traced from the 

 epidermis at one edge of the outgrowth right across to the other 

 side. The cells which are cut off, usually about three or four 

 in number, become suberized and thickened, but the imiermost 

 remain thin walled ; the radial walls however are usually of some 

 thickness. 



After the suberization of the cell walls has taken place the 

 thin-walled colourless cells lying beyond them, which are cut off 

 from their water supply, speedily lose their turgidity, collapse and 

 shrivel (fig. 16). 



