THIRD PHASE OF GROWTH. 



425 



sequent external form and segmentation of a'leaf becomes sketched out, and this then, 

 according to the mode and course of elongation in each case, becomes a branched, 

 divided, pinnate, lobed, or simply a toothed leaf. In the great majority of cases the 

 whole of the embryonic tissue of a leaf is finally transformed by elongation into 

 permanent tissue : the whole leaf is then completely developed. Nevertheless, cases 

 also occur where the apex of the leaf persists for a long time in the embryonic 

 condition, while its basal portions become elongated and fully developed. This 

 happens even in the case of some large pinnate leaves of Dicotyledons — e.g. Ailan- 

 ihus, Rolinia, &c. : after some time, however, the growing-point of the leaf in these 

 cases becomes transformed wholly into permanent tissue. In some Ferns, on the 

 other hand (Nephrolepis, Gleichenia, Mertensia, Lygodium), the ends of the leaves, or 

 of their lateral segments, remain in the em- 

 bryonic condition for years, so that a con- 

 tinual or periodically repeated lengthening 

 of the organ takes place by elongation just 

 as in perennial shoots. 



The growing-points of shoots, how- 

 ever, may also, on the contrary, become 

 wholly transformed into permanent tissue, 

 when they of course cease to be growing- 

 points : this is the case, for example, in the 

 formation of the thorns of Rhamnus ca- 

 thartica, Gleditschia,'&viA others. In the latter 

 instance they are branched shoots, which 

 only produce very small leaflets, and con- 

 sist at first of very delicate tissue, which 

 subsequently, however, lignifies to masses 

 hard as stone : the points of these thorns 

 are the previous growing-points, which 

 are now likewise lignified. 



All that has been said so far refers to 

 the relations of growth in the Vascular 

 plants and the majority of the Muscineae : 

 as already pointed out above, the same 

 processes of growth may also be recog- 

 nised even in the Algae and Fungi, though often only in a rudimentary form and 

 taking a much simpler course. Besides these, however, as was stated in the lectures 

 on Organography, entirely different types of growth also occur. These may both 

 be illustrated by a few examples. 



One of the most remarkable cases is found in those Algae the growth of 

 which is not accompanied by cell-divisions, and among which the Caulerpa 

 in Fig. 262 may serve as an example. The figure shows at once that we 

 have here to do with a plant the whole vegetative body of which is moulded on, 

 and therefore grows according to, exactly the same plan as that of a creeping 

 vascular plant. The creeping shoot-axis w s produces branched roots on its 

 lower side, and leaves containing chlorophyll on its upper side. As in the 



FIG. 26r. — A 5 young leaves ai Fastittacea satLva. 

 C of Levisticum o^ci7iale,/paiu!& ; ^ pinnse of second 

 order. 



