244 BOTANY OP CROP PLANTS 



organic compound of sulphur, allyl sulphide. The com- 

 pound is volatile to a high degree, and is broken down by 

 heat; consequently the onion is milder when cooked than 

 when raw. As a rule, white varieties are milder than yellow 

 and red kinds, although there are exceptions to this. 



Uses of Onions. — Onions are most commonly used as a 

 vegetable, but in many instances for flavoring purposes. 

 The small varieties such as Queen, B arietta, and American 

 Silverskin are used for pickling. The Egyptian (perennial 

 tree onion) and multipliers are valued for the production 

 of bunchers. It is considered that the allyl sulphide in 

 onions stimulates the flow of digestive juices and hence 

 they are often recommended for those having a tendency to 

 constipation. Again, on account of the small amount of 

 starch and sugar they contain, onions are made a part of 

 the diet of invalids who are not allowed starchy foods. 



ASPARAGUS 



Generic Description. — Members of the genus Asparagus 

 are all perennial plants with rather fleshy roots and short 

 rootstocks. From the latter, arise branching aerial stems, 

 which are sometimes annual, as in the common edible 

 asparagus (A. officinalis), or perennial as in A. laricinus, 

 one of the ornamental asparagi. The stems are erect or 

 cHmbing, in some instances {A . falcatus) reaching a distance 

 of 25 feet or more. The small leaf -like structures along the 

 stem, the so-called "leaves," are in reality modified stems 

 (cladophylls) (Fig. 98). They may be slender, as in com- 

 mon asparagus, or broad, as in Smilax. They are arranged 

 in clusters or whorls in the axils of the true leaves. The true 

 leaves (Fig. 98) are scales or spines, usually very small. 

 They subtend the branches. The flowers are solitary, in 

 small umbels or racemes and arise in the axils of the scales 



