CHAPTER IV. 



THE STEM. 



MORPHOLOGY. 



Just as the well-developed primary root may be regarded as a 

 descending cone with apex below, so may the primary stem be 

 looked upon as an ascending cone with apex above. This doubly- 

 conical shape is characteristic of gymnosperms and dicotyledons 

 (fig. 2). In monocotyledons, where there is no tap-root, the stem 

 is typically in the shape of a cone with apex downwards, which is 

 fastened in the soil by adventitious roots which proceed from it 



(fig- 4)- 1 



The stem must be looked upon as the most important part of 

 the plant, since it is the only one always present. Hair-structures 

 are by no means constant, leaves may be absent, as in duckweed, 

 where the little flat green expansions are stem structures, and 

 lastly, roots, in rare cases, are non-existent, as, for example, in a 

 kind of duckweed {Wolffia arrhiza), the body of which consists 

 solely of a minute cellular green disc. 



A stem not only bears, as a rule, secondary stems or branches, 

 which are similar to it in shape and structure, but also other 



1 This is a convenient place to remark that plants are classified into several 

 large groups, which are again subdivided, and so on, till at last we reach 

 species. These include individuals which differ only in slight features, such as 

 size, and are evidently descended from a common ancestor. The subdivisions 

 of plants in order of size are : group, division, class, subclass, series, cohort, 

 order (or family), genus, species. Flowering Plants form a group with two 

 divisions : (1.) Gymnosperms, including pine, fir, yew, juniper, larch, cypress, 

 &c. ; (2.) Aug io sperms, embracing most flowering plants, as, e.g., all those with 

 coloured flowers. This division has two classes : (a.) Monocotyledons (the 

 smaller division), taking in palm, arum, lily, tulip, onion, snowdrop, iris, 

 sedges, grasses, &c. ; (b.) Dicotyledons, to which most British forms may be 

 relegated, e.g., the majority of trees and shrubs, buttercup, wallflower, pink, 

 poppy, violet, waterlily, geranium, gorse, rose, daisy, dead nettle, primrose, 

 &c, &c. 



Every plant has, for the sake of identification, a double scientific name, 

 compounded of the names of its genus and species. For example, daisy is 

 Bettis perennis. Bellis — genus, perennis — species. 



