377 



if a yam be fxaniiiKMl. it will bo noticed that at the top the tissue 

 is rather hard and woodv : that hardish tissue is a \y<ivt of a i)ereiiiiial 

 underground "stem:" and it i)roducos the new annual shoot or 

 shoots, and new roots each successive yeai-. whereol' one. perhaps 

 more than one. l)econu's ileshv and swells u[i into the annual tuber. 

 This tuber is so built into the bit of stem, that it is ditfieult to tell 

 where the one ends aiul the other lieizins. and the intern .sometimes 

 sends up short shoots in the autumn which are themselves fleshy. 

 Ivaees vary among themselves much in this peculiarity, which as 

 it appears not to occur in any wild allies, must have stepped in 

 during man's period of modifying the plant: but it does not seem 

 clear how he would look upon it in selecting, unless as it spoils the 

 evenness of the tuber he would select against it. Yet there is the' 

 feature to he reckoned with. 



Just as unsought for would 1)C another development, which 

 hap])ens on the other haiul to have served man's purjtose, and to 

 have been sei /d on by him for the production of a curious group 

 of races wlr a do not bury Iheir tubers: the group has been as it 

 were side-t ./eked, and forms the development illustrated on plate 

 G. Assuredly in the ancestral condition, the yam plant sent its 

 succulent tuber deeply into the soil : and certainly if it had not 

 done so wild pigs and such animals would have destroyed it from off 

 the face of the earth : but after man had taken the yam into domesti- 

 cation and was protecting it, certain plants apparently lost the 

 tendency to send their fleshy root downwards. — they lost somehow 

 the responsiveness to gravity that they should have, and produced 

 tubers which, while elongating, still remained at the surface. Such 

 plants can be cultivated by earthing up : and they were pro!)agated 

 evidently as convenient for growing: they could for instance be 

 ])lanted in the midden at the back door, which grew with them. 

 These peculiar races deserve a special place in our classification, 

 which we mav start with a diagram as follows, side-tracking them : — 



1 short 



4 not burying 

 2 half-long 



3 long and deep burying 

 Under these four heads the yams will be considered. 



