466 LECTURE XXVIII. 



' flower ' from its last segments : I have myself confirmed this in various other 

 IMosses. According to Kühn and Leitgeb, moreover, the male organs or antheridia 

 of Andreaa and of the aquatic Moss Foniinalis arise in the same way as the 

 archegonia, so that the apex of the growing-point of the Moss may be transformed 

 into a female or into a male reproductive organ. We may, in addition, also regard 

 the formation of the sporangium of Mucor at the end of the simple utricular branch 

 as a case of the formation of an organ directly from the apex of the growing-point : 

 the end of the hypha, which functions as a growing-point, dilates into a sphere 

 and produces the spores in its interior. 



When in these and other cases the growing-point of a shoot is directly 

 transformed into a special organ, its previous growth in length is, as already 



mentioned, brought to an end. A limi- 

 tation of the latter may also take place, 

 however, simply by the embryonic tissue 

 becoming entirely transformed into per- 

 manent tissue, as in the case of ill- 

 nourished Fern prothallia, which then 

 dispense with their growing-point : with 

 a proper supply of food-materials, how- 

 ever, a new growing-point may arise. 

 The formation of thorns affords an 

 entirely different example of this class. 

 The hard point at the end of the small- 

 leafed shoot of Gleditschia, and in simi- 

 lar cases, is evidently the growing-point 

 transformed into lignified permanent 

 tissue. In other cases, again, the ac- 

 tivity of the growing-point simply comes 

 to an end ; it disappears, so to speak, 

 its tissue becoming merged into neigh- 

 bouring permanent tissue. This is the 

 case, for example, in many flowers with 

 a central ovary but without a central 

 ovule. Finally, the terminal bud of a 

 foliage-shoot, together with its growing-point, may periodically die off", the next 

 lateral bud continuing the growth of the previous axis : in this way are formed 

 sympodia, the stem of the Lime being an example. 



The peculiar characteristic of the growing-point becomes clear, however, in the 

 cases of so-called indefinite growth. Here the apex continues to grow undisturbed, 

 while underneath it (or, when the growing-point is flat, in circles around the central 

 apex) organs of like kind are produced in continuous succession ; these appear at 

 first as mere protuberances of the embryonic tissue, but become transformed later 

 wholly into permanent tissue. In this continuous repetition of products of like kind 

 from the circumference of the growing-point, we have one of the most universal laws 

 of growth of the whole vegetable kingdom ; organs of the most various kind—roots, 

 lateral shoots, sporangia, and sexual organs, may spring in this way in continual 



Fig. yi-^.— Rheum undulalum. Longitudinal sectior 

 of the flower bud ; s fi floral envelopes ; a .? anthers ; d 7 

 glands at base of stamens; y ovary ; n stigma; k k 

 nucellus of the ovule (slightly magnified). 



