4a 



LECTURE IV. 



situated, only grows very slowly in length, the growth of the young leaves is on the 

 other hand much more vigorous ; as each older leaf lying further back from the 



PIQ. j3._ Apical regions of two primary shoots of 2V-rt /1/a!.r. Apex 

 of the very small-celled growing point, from which the leaves *, *', *", 

 *'" arise as multicellular protuberances, which soon surround the stem, 

 and envelope it and the younger leaves like sheaths. In the axil of 

 the third youngest leaf i'. the very young rudiment of a branch is 

 visible as a roundish protuberance (cf. Fig. 5). 



FIG. 3i.— Longitudinal section through the 

 apical region of the primary stem of Helianlhus 

 annuus, immediately before the development of 

 the flowers, j apex of the broad growing 

 point ; *, b youngest leaves ; r cortex ; m pith. 



growing point has a start of all the younger ones, and as, at the same time, each 

 older leaf is arched concave inwards and envelopes the younger more or less, 



leaf thus lying on leaf, a bulb-like body, 

 the young shoot, is formed. If this 

 now becomes further unfolded, the leaves 

 expand according to their succession 

 in age, attain their complete develop- 

 ment, and become thrown outwards, 

 while at the same time the corresponding 

 segments of the shoot-axis are further 

 developed. Sometimes the bud consists 

 only of a few very young leaves, as in 

 the climbing-buds of the Ivy; in other 

 cases, however, dozens of leaves and 

 internodes are found in the bud state. If 

 the period of vegetation is interrupted 

 by the winter, the bud may maintain its 

 form, simply because the further de- 

 velopment of the parts is discontinued, 

 until, with favourable weather, the older 

 parts of the bud again begin to grow, 

 and new ones are formed at the growing 

 point ; e. g. in our so-called Acacia (RobiJiia pseudacacia) and the Arbor vitae 

 {Thuja). In the majoril}- of trees and shrubs, however, peculiarly organised winter- 



FIG. 35.— I-ongitudinal section of the fruit of Zea Mais (X 6). 

 c pericarp ; n remains of stigma \/s base of the fruit ; eg dense 

 yellowish portion of endosperm ; e-w whiter, less dense por- 

 tion of endosperm ; sc scutellum of embryo ; ss its apex ; e its 

 epithelium ; k plumule ; w (below) primary root ; -ws its root- 

 sheath ; -ui (above) secondary roots arising from the first inter- 

 node (J<) of the embryonal stem (cf. Fig. 27). 



