74 ELEMENTS OF BOTANY. 



100. Morphology of the Potato. — It is evident that in the 

 potato we have to do with a very greatly modified form of 

 stem. The corky layer of the bark is well represented, and 

 the loose cellular layer beneath is very greatly developed ; 

 wood is almost lacking, being present only in the very narrow 

 ring which was stained by the red ink, but the pith is greatly 

 developed and constitutes the principal bulk of the tuber. 

 All this is readily understood if we consider that the tuber, 

 buried in and supported by the earth, does not need the 

 kinds of tissiie which give strength, but only those which are 

 well adapted to store the requisite amount of nourishment. 



101. Structure of a Bulb; the Onion.^ — Examine the external 

 appearance of tlie onion and observe the thin membranaceous skin 

 which covers it. This skin consists of the broad sheathing bases of the 

 outer leaves which grew on the onion plant during the summer. Eemove 

 these and notice the thick scales (also formed from bases of leaves as 

 shown in Fig. 37) which make up the substance of the bulb. 



Make a transverse section of the onion at about the middle and sketch 

 the rings of which it is composed. Cut a thin section from the interior of 

 the bulb, examine with a moderate power of the microscope, and note 

 the thin-walled cells of which it is composed. 



Split another onion from top to bottom and try to make out : 



(a) The plate or broad flattened stem inside at the base. Fig. 36 a ; 



(6) The central bud ; 



(c) The bulb-scales; 



(d) In some onions (particularly large, irregular ones) the bulblets or 

 si'de buds arising in the axes of the scales near the base. Fig. 36 6. 



Test the cut surface for starch. 



Since the onion grOws so rapidly on beihg planted in the 

 spring there must be a large supply of nutritive material in 

 the bulb. Much of this is in the form of proteid material. 

 The proteids (§ 35) constitute a class of animal and vege- 

 table substances, very valuable for food, of which the whites 



1 ProbaWy a bulb with narrow scales like those of the lilies would be a more inter- 

 esting form for study, but the onion is always and everywhere obtainable. 



