34 SCIENCE PRIMERS. [vin. 



till the stem strikes a support, when the portion 

 above the point of contact with the support continuing 

 to revolve as it lengthens, naturally twines round and 

 ascends it. Such stems, if they find no support, 

 become weak as they lengthen, and fall on the ground. 



52. The principal underground forms of stem are 

 (a) The bulb, a very short, usually underground 



stem, with excessively crowded, overlapping leaves. 

 These leaves are wrapped round one another in the 

 onion, but simply overlap in the tiger-lily. 



(#) The rhizome or root-stock, a woody under- 

 ground stem, which sends root-fibres from its lower 

 side, and buds and leaves from its upper side (iris). 

 The corm is a very short fleshy rhizome (colchicum). 



(<:) Bulbils are small bulbs or corms formed at 

 the side of old ones, and are hence analogous to 

 branches, under which they will be further noticed. 



53. The tissues of the stem of flowering plants are 

 arranged on two plans, one characteristic of Dicotyle- 

 dons, the other of Monocotyledons (Par. 39). These 

 plans must be understood by the pupil, and can be so 

 by a little patience and practice with specimens ; they 

 are best illustrated by three such examples as the flax, 

 lime, and butcher's broom, or asparagus. 



54. The flax plant (a Dicotyledon) has an erect her- 

 baceous stem of many internodes (Par. 50), with leaves 

 all the way up, and flowers at the ends of the branches. 



A magnified cross-cut of the stem shows that it 

 consists of a cylinder of cellular tissue (Par. 6), traversed 

 vertically by a ring of wedge-shaped fibro-vascular 

 bundles (Par. 9), which are separated from one 

 another by the cellular tissue. The central cellular 

 tissue is the future pith, that at the circumference is the * 



