THE GROWING POINT 285 



extension ; (6) the region of completion. The first two regions only correspond 

 to our ' growing point', the remaining regions will be dealt with in the next 

 lecture.] Obviously, the growth periods pass gradually into each other, still 

 they differ sufficiently to justify a special treatment of the second and third 

 in the next lecture, just as we have dealt with the more important phenomena 

 presented by the first in the present lecture. A detailed discussion of the 

 growing point will be found in Sachs (1882), and Goebel (1884). 



Bibliography to Lecture XXII. 



[Berthold. 1904. Untersuchungen z. Physiologie d, pflanzlichen Organisation. 



Leipzig, 2, 185.] 

 Braun, a. 1 83 1. Nova acta acad. Leop. 15, I, 199. 

 Bravais. 1837. Annal. Sc. nat. II, 7, 42, 

 Bravais. 1839. Ibid., II, 12, 5. 



CoRRENS. 1899. Festschrift f. Schwendener. Berlin. 

 GoEBEL. 1880. Arb. d. bot. Inst. Wiirzburg, 2, 353. 

 GoEBEL. 1884. Vgl. Entwicklungsgesch. der Pflanzenorgane (Schenk's Handbuch 



der Botanik, 3). Breslau. 

 GoEBEL. 1898. Organographie, 1, 61. Jena. 

 Hansen. 1881. Abh, d. Senkenbergschen Gesell. 12. 

 HoFMEiSTER. 1 868. Allgcm. Morphologie d. Gewachse. Leipzig. 

 Noll. 1896. Sitzungsber. Niederrhein. Gesell. f. Natur- u. Heilkunde, 3. Febr. 



1896. 

 [Noll. 1903. Biol. Centrbl. 23, 281.] 



Reinke. 1899. Ueber Caulerpa (Wiss. Meeresunters. Kiel, N. F. 5, i). 

 Sachs, J. 1878 and 1879. Arb. d. bot. Inst. Wiirzburg, 2, 46 and 185. Gesam. 



Abhdl. 2, 1067, &c. 

 Sachs, J. 1882. Vorles. iiber Pflanzenphysiologie, p. 939. Leipzig. 

 Schwendener. 1878. Mechan. Theorie d. Blattstellungen. Leipzig. 

 Seckt. 1 90 1. Bot. Centrbl. Beihefte, 10. 

 Westermaier. 1 88 1. Jahrb. f. wiss. Bot. 12, 439. 



LECTURE XXIII 



ELONGATION AND INTERNAL DIFFERENTIATION 



We have learned to recognize in the continued activity of the growing 

 point an essential difference between the plant and the animal. In the latter 

 the primordia of all the organs are laid down in the embryo, and growth con- 

 tinues for long afterwards, often for many years, during which the enlargement 

 of these embryonic primordia unto the fully adult condition takes place, but 

 a jresh formation of new organs from a persistent embryonic substance occurs 

 only in plants. We might say, indeed, that a normal plant is never full grown, 

 but consists of fully grown parts coupled with parts having a capacity for further 

 development. This difference between an animal and a plant is, however, not 

 so thoroughgoing as one might at first sight suppose. 



Growth, as we have seen, may be restricted to the growing point, so that 

 the axis attains its prescribed length and thickness at quite a short distance 

 behind the active apex, and the rudiments of the lateral organs are placed at 

 the same intervals apart that they maintain later on. In other cases apical 

 growth is only feebly developed, and the organs which arise from the growing 

 pomt frequently exhibit, at a certain distance from it, conspicuous increase 

 both in length and in thickness. It is with these phenomena of elongation that 

 we have to deal in this lecture. 



