1010 SUMMARY OF CURRENT RESEARCHES RELATING TO 



formation of an entirely new layer of tissue, the periderm. The 

 healiug of the wound may take place in three different ways, either 



(1) by desiccation of the wounded surface, as in tree-ferns; or (2) by 

 the formation of cells of a reticulated-fibrous character, as in Orchideae; 

 or (3) by the formation of periderm. 



The formation of periderm is by far the most common process 

 in the healing of wounds. It occurs in all perennial plants, and is 

 one of the most widely diffused examples of secondary growth in the 

 vegetable kingdom. In the case of the fall of leaves the layer of 

 periderm formed in this way is readily reeognized by its brown 

 colour. 



Assimilating System of the Stem.* — Herr A. Nilsson classifies 

 the variations in the assimilating tissue of the stem under three types, 

 viz. — (1) The assimilating system consists only of assimilating tissue; 

 i. e. this latter serves as well both for conduction and abduction. 



(2) The system consists of assimilating tissue and abducting tissue. 



(3) It consists of assimilating tissue, conducting tissue, and abducting 

 tissue. A number of subordinate classes are arranged under each of 

 these types. Light is an important factor in determining the develop- 

 ment of the assimilating tissue, the palisade-cells being almost entirely 

 dependent for their formation on a strong illumination. 



Comparative Anatomy of the Stem of Dicotyledons-! — M. J. 

 Herail has compared the structure of the stem in a large number of 

 families of Dicotyledons. The following are the more important 

 general results : — 



In the primary cortex of Buxaceae there are found isolated 

 fibrovascular bundles, which anastomose at the nodes with those of 

 the central cylinder ; and the same is the case in the Leguminosaa 

 (Viciese), where they are in connection with the leaves. The primary 

 cortex of certain Melastomacess incloses fibrovascular bundles which 

 appear to have no connection with the leaves ; all the plants of this 

 family have also medullary bundles. The Menispermaceae have, at 

 first, the typical structure of the stem of Dicotyledons ; at a later 

 period fibrovascular bundles appear in the primary cortex of some 

 species, which may be climbing or not. The same is the case in the 

 Schizandreae, Lardazibaleae, certain Leguminosae, and Aristolochiaeeae ; 

 but they have no connection with the habit of the species. They are 

 arranged in continuous circles round the central primitive cylinder. 



Departing from its ordinary functions, the pericycle is often 

 the seat of an exceptional development of vascular bundles ; ex- 

 amples occur in the Calycanthaceae, Chenopodiacese, Pbytolaccaceaa, 

 Nyctagineae, and Aizoaceae. 



The pith very often contains abnormal structures, especially 

 vascular bundles. In the Cucurbitaceae these are bicollateral, all the 

 tissues having the same origin, exclusively primary ; but elsewhere, 

 in the Solanaceae, Cestrineas, Nolaneae, Apocynaceas, Asclepiadeae, 



* Naturv. Students'allsk. Upsala, March 30, 1886. See Bot. Centralbl., xxvii. 

 (1886) p. 27. 



t Ann. Sci. Nat. (Bot.), ii. (1885) pp. 203-314 (6 pis.). 



