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. At7an- 

 thus, Robinia, &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, Gleichema, 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- 

 tharUca, GleditscMa,2LX\A 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 : c 

 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 diff'erent types of growth also occur. These may both 

 be illustrated by a few examples. 



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

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

 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 v s produces branched roots on its 

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



lit; leaves of Pastiiiacea : 

 T/t-,y pinna; ; 1^ pinnae of: 



