LEAVES AND LEAF-FORMING AXES. 



"^33 



ceases or becomes weaker at the apex, and when, at the same time, an active growth 

 continues in a transverse zone or at a place beneath the apex, that new leaves can 

 become interpolated between those already in existence ^ 



(5) T/ie Leaves ahvays originate from the Primary IMeristem of the Piinctum 



Fig. 107. — Apicalrcgionof two primary shoots of maize. Apex 

 of the very small-celled vegetative cone, out of which the leaves 

 b, b' , f>", b'" arise as multicellular protuberances, which soon 

 embrace the stem, and envelope it and the younger leaves like 

 a sheath. In the axil of the third youngest leaf b" the youngest 

 rudiment of a branchlet is visible as a roundish protuberance. 



KiG. 108. — Longitudinal section of the apical region of the pri- 

 mary stem of the sunflower, immediately before the formation of the 

 flowers; a the apex of the broad punctum vesetationis ; bb the 

 youngest leaves ; r the cortex ; w the pith. 



VegetationiSf never from those parts of the stem which already consist of fully 

 differentiated tissues. In Characece, 

 Mosses, &c., before or during the first 

 divisions of their segments the leaves 

 become visible close beneath the apical 

 cell, as protuberances, the outer portion 

 constituting an apical cell, out of the 

 segments of which are formed the leaves. 

 In Vascular Cryptogams a many-celled 

 vegetative cone often overtops the young- 

 est rudiment of a leaf {e.g. strong Equi- 

 setum buds, Salvinia, many Ferns and 

 Selaginellae). In Phanerogams (Figs. 

 107, 108, 109) this is general; in them 

 the rudiment of the leaf does not begin 

 with an apical cell standing out beyond 

 the vegetative cone, as in Cryptogams, 

 but a roundish or broad cushion is 



formed, which from its very first origin consists of numerous small cells capable 

 of division. 



(4) The Leaves are always Exogenous Formations, i. e. the rudiment of the 



Fig. 109.— Longitudinal section through the apical region of 

 an upright shoot of Hippttris vulgaris ; s the apex of the stem ; 

 b, b, b the leaves (verticillate) ; k A the buds in their axils, which 

 all develope into flowers ; g^^r the first vessels (the dark parts of 

 the tissue indicate the inner rind with its intercellular spaces). 



' Since phenomena of this kind are confined to the flowers and inflorescence of Phanerogarp.":^ 

 their consideration may for the time be postponed. 



