THE STEM. 23 



as in the case of the root, may be regarded as slender cones 

 attached to the side of the primary one. Here, too, their suc- 

 cession is regular and acropetal. Adventitious stems may also 

 be formed, and this is the nature of most of the slender twigs 

 which often sprout from old tree stumps. 



The majority of stems are aerial, but water stems, partly or 

 entirely submerged, are naturally present in marsh and water 

 plants. Aerial stems are sometimes parasitic, as in clover- dodder, 

 which possesses suckers on its stem that become intimately united 

 with the stem of the clover. It is commonly imagined that all 

 underground structures are roots. This, however, is a mistake, 

 since a great many stems are more or less subterranean. 



The external form of a stem is influenced by a number of 

 factors. The most obvious is that of size and duration. Upon 

 this depends the old and somewhat vague classification into herbs, 

 shrubs, and trees, as well as the division already mentioned into 

 annuals, biennials, and perennials. 



Herbs are plants, usually of no great size, in which either the 

 whole plant dies after a year's growth (annual herbs), or else the 

 subaerial portions die away every year (biennial and perennial 

 herbs). The stem is not at all or scarcely woody. Shrubs are 

 perennials, mostly less than twenty feet high, with woody stems 

 that branch from or near the ground. Trees are perennials, 

 generally more than twenty feet high, with a distinct woody 

 primary stem or trunk. There are, however, no sharp boundaries 

 between these divisions. Herbs pass into shrubs, the intermediate 

 small woody perennials being called undershrubs ; and shrubs, 

 again, are connected by all possible gradations with trees. The 

 names herb, &c, refer to the shape of the overground parts of the 

 plants taken as a whole. The form of the individual stem is, in 

 the majority of cases, cylindrical and solid. Some herbs, how- 

 ever, possess square or triangular stems, while ribbed and flattened 

 forms are not uncommon. It may happen that the last kind are 

 winged, or produced at the sides into thin green expansions. 

 Hollow or fstular stems are often found among herbs, growth in 

 thickness having exceeded increase in substance, the result being 

 that the internal tissues have been ruptured. This is very typi- 

 cally seen in grasses, where the tubular internodes are separated 

 by swollen nodes. Bamboo and any kind of straw will serve as 

 examples. These stems are called culms. Not only may the size 

 of the whole stem system be considered, but also the sue of the 

 individual parts as regards length and thickness. The maximum 

 length is attained in the trunks of trees, where, as in all long 

 stems, the internodes are well developed. Great development of 

 the internodes may cause the stem to be thin and weak, as in 



