28 MOEPHOLOGT, OR COMPARATIVE ANATOMY. 



and forms a new corm, the old one shrivelling up. The leaves appear 

 above ground in the spring 1 , proceeding from the apex of the corm, and 

 the bud at the side of its lower end shoots out to form a new lateral stem, 

 which produces sheaths and rudimentary leaves, and ultimately forms the 

 flowering stem of the next autumn, the base of which repeats the forma- 

 tion of a corm in like manner and shoots up its tuft of leaves in the fol- 

 lowing spring. The corm being formed from the internode between the 

 buds, the lower of these is, to a certain extent, basilar as well as lateral, 

 while the upper one appears near the top of the perfect corm, rather to 

 one side, near the scar of the old leaves and flower-stalk : this bud may 

 or may not be developed into a corm simultaneously j but in any case 

 it becomes detached from its fellow when the old corm shrivels up, and 

 thus may multiply the plant. 



The corm of Arum maculatum, examined in spring, exhibits two lobes, 

 with an intermediate constriction; they lie adjoined horizontally : the 

 corm of the past year is shrivelled ; the other is solid, and at the summit 

 exhibits sheathing scales enveloping the base of the erect flowering stem. 

 Opening the sheath, which turns upward, we see that the flower arises 

 from a terminal bud, while in the axil of a leaf arising below it exists a 

 bud which is destined to swell up and form a new corm for the next 

 season, the oldest one meantime withering away ; so that two genera- 

 tions with the rudiments of the third always coexist ; these generations 

 may consist of a greater number of individuals when additional cornis 

 arise from the axils of several of the scales of the parent corm. 



Tubers. The stem-tuber is either formed Fig. 19. 



from the base of a stem, or from a branch 

 arising from a subterraneous leaf-scale (fig. 

 20), developed either partially or entirely into 

 a thick and fleshy mass, by expansion of its 

 spongy structure, its own leaves appearing 

 in the form of rudimentary scales, in the axils 

 of which exist dormant buds, or eyes, capable 

 of producing independent stems when the 

 tuber recommences its development after a 

 season of rest. 



Axial tubers occur in many herbaceous plants, 

 as in Corydalis bulbosa ; when of annual dura- 

 tion, these are essentially the same as conns. 

 The tuber of the Potato is a familiar example of 

 the stem-tuber formed from a branch, in which its 

 characters may be readily observed ; a number of 

 leaf-scales at the base of the "haulm" send out 

 subterraneous branches, which at some distance 

 from the point of origin cease to elongate, and 

 swell up into tuberous masses. The tubers of the Jerusalem Artichoke 

 (fig. 20) are analogous productions. Stem-tubers passing more or less 

 into rhizomes form the so-called roots of the Bryonies (Tamus communis 

 and Bryonia dioica), of the Sweet-potato (Convolvulus Batatas), and the 



