WEEDS. 59 



CHAPTER V. 

 WEEDS. 



Plants, according to their manner of growth, are either 

 exogenous (outside growers), their bulk being augmented 

 by layers next to the inner bark; or endogenous (inside 

 growers), which increase from the centre, the new growth 

 pressing the older outwards. 



All the trees of the United States, with the exception 

 of the Palmetto, are outside growers. The Asparagus is 

 an example of an inside grower. 



The stem of an outside grower consists of the outer 

 bark, the inner bark, or liber, the sapwood or alburnum, 

 the heartwood and the pith. The medullary rays connect 

 the pith horizontally with the inner bark through the 

 wood. 



When a seed germinates, it sends its radicle down into 

 the ground and its sprout (plumule) up into the air. The 

 root obtains from the soil crude sap, containing solu- 

 tions of gases, earths, and salts, which ascend through 

 the sapwood to the leaves. Leaves may be considered a 

 continuation of young bark, and in plants, which are 

 naturally leafless, the young bark performs their office. 

 Leaves are provided on their upper and lower surfaces 

 with breathing pores, or stomata; and those plants grow- 

 ing in moist and shady places, are most abundantly 

 supplied with them, and the pores are of larger size. 

 The outer skin, or epidermis, of thick-leaved plants 

 growing in hot, arid situations, like the aloe, the prickly 

 pear, and the purslane, have few and small breathing 

 pores, and the skin is still further protected by a waxy 

 covering. When the sap reaches the wide expanse of sur- 

 face of the leaves, it comes in contact with air, heat, and 

 light, the crude sap is digested and the excess of water 



