YEAST 0¥ ECONOMIC PLANTS. 445 



Yam, OMnese (Dioscorea latatas), a native of China and 

 Japan, where it is extensively cultivated. It diifeis from the 

 preceding in having a spindle-shaped, black root, about the 

 size of a parsnip, 2 to 3 feet long. It has been introduced into 

 this country, and is perfectly hardy ; indeed at one time 

 expectations were entertained that it might prove a good substi- 

 tute for the potato ; but so long as potatoes are to be had it 

 will not find much favour, being far inferior to them as a 

 vegetable. 



Yangmae, a name in China for the fruit of Myriea Nagi, a 

 small tree of the Candleberry Myrtle family (Myricacese), native 

 of China. In the island of Chusan it is described by Mr, 

 Fortune as a bushy shrub or tree, 15 to 20 feet in height, and 

 when he saw it, " it was loaded with a dark-red fruit, not very 

 unlike the fruit of the strawberry tree (Arbutus), but much, 

 larger. There was also a variety with yellowish fruit. The 

 natives were busily engaged in gatheimg the fruit, and packing 

 it in baskets for the markets. The gatherers offered me liberal 

 supplies of this fine fruit." 



Living plants were introduced into this country in 1844, but 

 it Iras not received the patronage of fruit -cultivators that it 

 seems to merit, if. Nagi is also known in Western India, but 

 its fndt is very inferior to that of the Chinese. It is probable 

 they are two distinct species. 



Yari-Yari, a name in Guiana for Lancewood (which see). 



Yeast, or Bann, a name given to tlie frothy scum that forms 

 on the surface of fermenting sugary liquids, and rises from the 

 bung-holes of barrels of newly-brewed beer. The microscope 

 shows that this froth consists of particles which multiply with 

 extraordinary rapidity when placed in a moderately-waim 

 temperature. By experiments it has been ascertained that the 

 particles or globules germinate ; and they are considered to be 

 the spores of a mould fungus, belonging to the genus Torula, the 

 spores of which are but slightly united, the mycelium being 

 almost absent. Yeast is not only employed in hastening the 

 fermentation of worts, it also is an important agent in leaven- 



