28 BOTANY PART i 



other species of the genus, e.g. Am^dopsis Veitddi (Fig. 29), the 

 tendrils are able to form adhesive discs at the tips of the branches, 

 and thus to cling to flat supports. 



Stem-thorns. Shoots may undergo a still greater reduction by 

 their modification into THORNS, as a defence against the depredations 

 of animals. Of shoots modified in this manner, the Black Thorn 

 (I'm a us spinosa), the White Thorn (Crataegus), and the Honey Locust 

 (Gleditschia) afford instructive examples. The thorns are simple or 

 branched, hard, pointed bodies. In Gleditschia (Fig. 30) the thorns 

 are developed primarily from the uppermost of several serial buds ; 

 while secondary thorns may develop on older portions of the stem 

 from the lower buds of the series, and thus give rise to clusters of 

 thorns. In Colletia cruciata all the shoots are flattened and spiny. ><> 

 that they perform the duties of the leaves, which are quickly lost, in 

 addition to serving as protective structures. This plant is an American 

 shrub belonging to the Rhamnaceae, and grows in dry sunny situations. 



Flowers. The most marked changes in the form of the shoot, due 

 to the displacement and union of its different members, take place in 

 phanerogamic flowers (Fig. 39). The shoots from which flowers are 

 developed are termed FLORAL SHOOTS, in contrast to the FOLI.MJK 

 SHOOTS, the functions of which are merely vegetative. The axis of 

 the floral shoot remains short, and becomes flattened or even depressed 

 at the tip. The leaves which spring from the floral axis often become 

 united with one another and with the axis itself. In such cases 

 thorough investigation of the development and the comparative 

 morphology of the flower is necessary to reveal the modifications 

 which have taken place during its evolution. In most instances the 

 rule seems to hold that axillary buds are not formed within a flower 

 except in cases of abnormal development. 



The Order of Sequence of Shoots. If the vegetative cone of the 

 primary axis of a plant, after reaching maturity, is capable of repro- 

 duction, a plant with but one axis will result, and the plant is 

 designated UNI AXIAL or HAPLOCAULESCENT. Usually, however, it is 

 not until a plant has acquired axes of a second or third order, when it 

 is said to be DIPLOCAULESCENT or TRIPLOCAULESCENT, or of the nth 

 order, that the capacity for reproduction is attained. A good illus- 

 tration of a plant with a single axis is afforded by the Poppy, in 

 which the first shoot produced from the embryo terminates in a flower, 

 that is, in that organ of Phanerogams which includes the sexual 

 organs. As an example of a triplocaulescent plant may be cited the 

 common Plantain (Plantago major), whose primary axis produces only 

 foliage and scale leaves ; while the secondary axes give rise solely to 

 bracteal leaves, from the axils of which finally spring the axes of the 

 third order, which terminate in the flowers. In the case of trees, 

 only shoots of the nth order can produce flowers. Thus a division of 

 labour commonly occurs in a branched plant, which find-; its expression 



