154 



MORPHOLOGY OF MEMBERS. 



Point, never from those parts of the stem which already consist of fully differentiated 

 tissues. In Characeae, Muscineae, &c., before or during the first divisions of their 

 segments, the leaves become visible close beneath the growing point as protuber- 

 ances, the outer portion constituting an apical cell, from the segments of which a 

 leaf is built up. In Vascular Cryptogams a many-celled cone of growth often over- 

 tops the youngest rudiment of a leaf, as in strong Equiseiwn buds, Salvima, many 

 Ferns and Selaginelleae. In Phanerogams (Figs. 117, 118, 119) this is general^ ; in 

 them the rudiment of the leaf does not begin with an apical cell projecting from 

 the cone of growth, as in Cryptogams, but a rounded or broad cushion is formed, 

 which from its very first origin consists of numerous small merismatic cells. 



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

 leaf never has its origin exclusively in the interior of the tissue of the stem, and 

 is never covered by layers of tissue of the stem which take no part in its formation, 



Fig. 117. — Terminal region of two primary shoots of maize. 

 Apex of the very small-celled cone of growth, out of which the 

 leaves b, b' , b', 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 young- 



est rudiment of a branchlet is visible as a roundish protuberance. leaves ; r cortex ; m pith. 



Fig. 118.— Longitudinal section through the apical region of the 

 primary stem of the sunflower, immediately before the formation of 

 the flowers ; j apex of the broad growing point ; b b the youngest 



as is the case with roots and many endogenous shoots. In Cryptogams it is usually 

 a single superficial cell {i. e. superficial before the differentiation of the epidermis) 

 which forms the foliar protuberance. In Phanerogams a mass of tissue bulges 

 out as the rudiment of the leaf, and consists of a luxuriant growth of the periblem 

 covered by dermatogen (Sect. 19, Fig. 113, p. 147). By this means the leaf is at 

 once distinguished from the hair even in its most rudimentary state. The hair is an 

 outgrowth of the epidermis; but since in Phanerogams the primordial epidermis 

 or dermatogen covers the whole of the growing point above the leaves, hairs may 

 also spring up higher in position than the youngest leaves, from single cells 



^ [Warming however remarks (Ramification des Phanerogames, p, iii) that the growing point 

 may have the most various forms, from that of a rather acute cone, as in Graminese, Amaranthus, 

 and Plantago, to that of a cup-shaped depression, e. g. Digitalis, and that the form may differ even in 

 species belonging to the same genus ; thus in Digitalis Intea it is convex, in D. parvijlora con- 

 cave.] 



