102 



SPINES OR THORNS. 



are easily detached, and are capable of producing young plants when 

 placed in favourable circumstances. 



199. Occasionally leaf-buds are produced naturally on the edges 

 of leaves, as in Bryophyllum calycinum and Malaxis paludosa (fig. 

 214), and on the surface of leaves, as in Ornithogalum thyrsoideum 

 (fig. 215). These are capable of forming independent plants. Similar 



buds are also made to ap- 

 b ~ * > ., pear on the leaves of Gesne- 



ra, Gloxinia, and Achime- 

 nes, by wounding various 

 parts of them, and placing 

 them in moist soil ; this 

 is the method often pur- 

 6 sued by gardeners in their 

 propagation. The cellular 

 tissue near the surface of 

 plants, seems therefore to 

 214 215 have the power of develop- 



ing abnormal leaf-buds in certain circumstances. Even roots, when 

 long exposed to the air, may thus assume the functions of stems. 

 Leaves bearing buds on their margin, are called proliferous (proles, 

 offspring, and jfero, I bear). 



200. Spines or Thorns. Branches are sometimes arrested in their 

 development, and, in place of forming leaves, become transformed into 

 spines and tendrils. Spines or thorns are undeveloped branches, ending 



in more or less pointed extremi- 

 ties, as in the Hawthorn. Plants 

 which have spines in a wild state, 

 as the Apple and Pear, often lose 

 them when cultivated, in conse- 

 quence of their being changed 

 into branches; in some cases, as 

 in Prunus spinosa, or the Sloe (fig. 

 217), a branch bears leaves at 

 its lower portions, and terminates 

 in a spine. Leaves themselves 

 often become spiny by the har- 

 dening of their midrib or primary 

 21? veins, and the diminution or 

 absence of parenchyma, as in Astragalus massiliensis (fig. 217 r), 



Fig. 214. Extremity of a leaf, !, of Malaxis paludosa, the margin of which is covered with 

 adventitious buds, 6 b; thus becoming proliferous. 



Fig. 215. Portion of the blade of a leaf, /, of Ornithogalum thyrsoideum, on the surface of 

 which are developed adventitious or abnormal buds, 6666, some of which are large. 



Fi<r. 216. Branch of Prunus spinosa, or Sloe, with alternate leaves, and ending in a spine or 

 thorn. 



Fig. 217. Pinnate leaf of Astragalus massiliensis, the midrib of which, r, ends in a spine .< 

 Petiolary stipules. /, Nine pairs of leaflets. 



