14 
LIFE HISTORY AND RACE HISTORY 
From its earliest stages the seedling has three primary organs, root, 
stem and leaf. It seems to be a rule that nature seldom makes a new 
organ when an existing organ can be modified to do the work in hand. 
Nearly all vegetative organs are, thus, considered to be modifications of 
these simple primary organs, root, stem and leaf. In the life history 
of plants we find development of a great variety of structures. In its 
adaptations to an endless combination of factors in its habitat the plant 
has variously modified its primary organs, root, stem, and leaf for varied 
purposes. The root is used for storage, as in the case of the Soap Plant 
(Chenopodium californicum) or the Man Root (Echinocystis fabacea). 
The stem, which is normally a - supporting structure for the display of 
foliage leaves, is variously modified. It becomes an underground struc¬ 
ture, such as a rootstock in the case of Scare-weed (Stachys bullata) 
or Telegraph-Plant (Pleterotheca grandiflora), structures by which the 
plant is enabled to retire below ground at unfavorable seasons and at the 
same time multiply in extent. Similarly we have runners, as in the case 
of the Beach Strawberry (Fragaria chilensis). which are stems modified 
for reproduction. Other stems are modified for purposes of storage as 
in the case of the corms of Blue Dick (Brod’aea capitata) and Grass 
Nut (Brodiaea laxa), or as in the case of the tubers of Potato (Solanum 
tuberosum). Sometimes, again, stems are replaced by thorns as' in 
Osage Orange (Madura pomifera) or tendrils, as in the species of 
Man-root (Echinocystis fabacea). These various modifications of a 
typical stem are to be determined as stem structure by observing that 
they have regular nodes and internodes, or bear buds, or bear leaves, or 
show unmistakably by their structural position that they are stems. 
The earliest foliage leaves borne on the stem of the seedling are almost 
universally simple, that is, with a single or undivided blade. A com¬ 
pound leaf is one in which the blade is divided up into tw r o or more 
parts or leaflets; it is regarded, within a given group, as being a develop¬ 
ment from the simple leaf. In the life history of those species that bear 
compound leaves, simple leaves at an early stage precede compound 
leaves, and it is held that simple leaves preceded compound leaves in the 
race history. When the leaflets are borne along the axis or prolonga¬ 
tion of the petiole, the leaf is called pinnate. When such divisions or 
leaflets are borne at the top of the petiole the leaf is called palmate. In 
some cases it is capable of demonstration that palmate leaves are an 
advance upon the pinnate type, being derived from the latter, as in the 
case of Potentilla, where in most species pinnate leaves are regarded as 
primitive and the palmate leaves of certain species as derived. 
Of the three primary organs the leaf is the more variously modified. 
The beginning student is likely to think of the leaf as solely a physiologi¬ 
cal mechanism, as an organ for photosynthesis, and use the term leaf 
when he means, not leaf, but foliage leaf. As highly important as is the 
foliage leaf to the existence of the plant it is after all, morphologically, 
only one kind of leaf. Leaves are modified for storage as in the Tiger 
Lily (Lilium pardalinum) or Checker Lily (Fritillaria lanceolata). They 
are modified in whole or in part for climbing, as tendrils in the case of 
Giant Vetch (Vicia gigantea). Sometimes again leaves are replaced by 
