162 MINUTE STEUCTDEE OF THE LEAF. 



ing water. On the external surface its thin-walled cells are in 

 close contact (there being nothing answering to stomata) ; but 

 in the interior of the leaf there are often lacunae filled with air. 

 These were thought bj- Brongniart to be essentially the same as 

 those cavities found in the parenchyma of msnay marsh plants. 



The veins of submerged leaves have no true ducts ; the elon- 

 gated fascicles generally consisting merely of rows of elongated 

 cells. ^ 



455. Roots may be produced from leaves in much the same 

 way as they are from stems ; that is, some of the cells at the 

 liber maj- divide in such a manner as to form a protuberance 

 which pushes before it a part of the endodermis. As the root 

 thus formed emerges, the tissues are speedily produced, the wood 

 being continuous with the wood of the leaf, the liber with its 

 liber. Eoots maj' arise naturallj' in some leaves by simplj- plac- 

 ing them in contact with moist earth, or they may be produced 

 artificially by mutilation of the petiole or lamina. Bryophyllum 

 cal^cinum affords a good example of the former ; Begonia, 

 Peperomia, etc., of the latter mode of origin. 



456. Buds may form spontaneously on the margin of leaves, 

 especially those in contact with a moist surface, or they may 

 grow from the cells under the scar where a mutilated leaf has 

 healed. 



457. In some of these cases only the epidermal cells take 

 part in producing the meristcm from which the bud is developed : 

 in others the parenchj'ma just below the epidermis also divides, 

 or the cells under the scar may produce all the axial tissue ele- 

 ments. Begonia is an example of the first method of production. 

 Bryophyllum of the second, Peperomia of the third. 



It is interesting to observe that in all these cases the bud forms 

 without the intervention of tlie fibro-vascular bundles of the leaf. 

 Tlie newly formed axis has flbro-\'ascular bundles, which may 

 anastomose with those pre-existent in the leaf, but usually they 

 are entirely distinct. The axis is, however, provided with its 

 own rootsystem, and after a time it becomes severed by a plane 

 of cork from the leaf which produced it. 



458. Fall of the leaf. In deciduous plants the leaf separates 

 from the stem or twig by the formation of a plane of cells '' 

 cutting sharply through the petiole at or A^ery near its base. 

 The dividing plane may be partially formed early in the growing 



1 Brongniart : Ann. des Sc. nat., tome xxi., 1830, p. 442. 



2 Called by Mohl the separative layer (Botaiiische Zeitung, 1860, p. 1). 



