736 THE FLORAL STEM. 



The stems here considered are protected against the lateral pressure by a layer 

 of thick-walled parenchyma (183^), or by the strands of tissue crossing the larger 

 air-canals which run longitudinally outside the circle of vascular bundles in the 

 stem (183^). In the underground stems of the Grass of Parnassus (Parnassia 

 palustris), and of several other herbaceous plants, there is no pith, they exhibit 

 a central strand of compressed vascular bundles and their structure is very similar 

 to that of roots growing in the ground. 



From this general account it is sufficiently evident that the arrangement of 

 the tissues in stems does not so much depend upon whether the part in question 

 belongs to a scaly stem, a foliage stem, or a floral stem, but rather upon its 

 relations with the outer world, and in particular upon the influences exercised by 

 the surroundings serving as a support or substratum. The stem, as the bearer of 

 the foliage and flowers, must be so constructed that the organs named may be 

 raised into the air, sunned, exposed to the wind and to the visits of flying insects 

 and birds, and retained in the most advantageous posture in spite of all opposing 

 influences of the environment. In such a stem are comprehended the various 

 organs of food-conduction, the conducting capacity of which must not be im- 

 paired by pressure, flexion, or strain. All the functions of the stem are influenced 

 and governed in a variety of ways by the varying circumstances of the habitat, 

 and by the forms of foliage and flowers peculiar to each species. These functions 

 are wonderfully correlated, and the diflerent arrangement of the tissues in the 

 stem in each individual case is nothing but the expression of the relation of the 

 form to the conditions under which the plant lives. 



THE FLORAL STEM. 



The portion of the stem from which floral leaves proceed is called the floral 

 stem (thalamus). It has the form of an axis, from the upper part of which project 

 the carpels and stamens, and below these the perianth leaves. The floral stem, like 

 every other, is built up of internodes whose number corresponds to the number of 

 leaves on its circumference, standing vertically above one another; but since the 

 vertical intervals are usually very small, the articulation of the stem is but seldom 

 plainly visible to the naked eye. Below the perianth leaves only the floral stem 

 appears more or less extended, and this portion is distinguished as the "flower- 

 stalk" from the part which bears the perianth leaves, which is termed the " floral 

 receptacle". 



The flower-stalk (pedunculus) originates only in a few Rafflesiaceae immediately 

 from the tissue which represents the scaly stem. It is also of comparatively rare 

 occurrence (restricted to a few annuals) that the stem proceeding from the bud 

 of the hypocotyl (i.e. the main axis of the whole plant) passes directly into the 

 flower-stalk and terminates in a floral receptacle. The flower-stalk often springs 

 as a lateral shoot from the main axis of the plant, and generally it proceeds as a 

 lateral axis from a stem structure which is itself only a lateral axis of the main 



