72 BOTANY PART i 



mass serve both equally ; usually certain parts are concerned with the 

 nutritive processes and others with reproduction. There is thus 

 usually a clear division of labour between the vegetative organs and 

 the reproductive organs, which are fundamentally different in form 

 and structure as well as in function. These two groups of organs will 

 require separate consideration. 



III. Relations of Symmetry. The form of the whole segmented 

 or unsegmented organism and of its parts is determined by their 

 relations of symmetry. Like nearly all properties of organic forms 

 this is closely connected with the mode of life of the organism, 

 especially with the direction of growth of the plant and the position 

 of its members in space. As a rule, therefore, the symmetry of the 

 internal construction of a plant corresponds to that of its external 

 form. 



Apart from a few very simple forms, the plant body and its 

 individual parts nearly always exhibit POLARITY and a distinction of 

 base and apex. Such a distinction is shown both in free motile 

 forms, in which the direction of progression is usually determined by 

 the polar construction of the body, and in attached forms, where the 

 organism is attached to the substratum by its basal pole. 



Every section through a part of a plant parallel to the longitudinal 

 axis is a longitudinal section. When it passes through the axis it is 

 termed a radial longitudinal section, and when it is at right angles to a 

 radius but not in the plane of the axis itself a tangential longitudinal 

 section. Sections at right angles to the long axis are transverse 

 sections. An organism or a part of a plant which is almost similarly 

 constructed around its longitudinal axis is termed RADIAL or ACTINO- 

 MORPHIO (Fig. 525 A). Such a structure can be divided by a number 

 of radial longitudinal sections into approximately equal halves, which 

 are mirror images of one another ; it has thus a number of PLANES 

 OF SYMMETRY. When there are only two planes of symmetry 

 standing at right angles to each other the structure is called 

 BILATERAL (Fig. 107). Lastly, when there is only a single plane of 

 symmetry (the MEDIAN PLANE) the structure is DORSIVENTRAL or 

 ZYGOMORPHIC ; the two lateral halves correspond, while the anterior 

 and dorsal sides are unlike (Fig. 525 B). Plants or parts of plants 

 which grow vertically upwards or downwards (ORTHOTROPOUS) are 

 usually radial or bilaterally symmetrical. When, on the other hand, 

 they grow oblique or at right angles to the vertical (PLAGIOTROPOUS) 

 they are frequently dorsiventral. There are also ASYMMETRICAL 

 organic structures, which cannot be divided by any plane into corre- 

 sponding halves. Some dorsiventral structures, e.g. leaves, become 

 asymmetrical by the one half being differently formed to the other. 

 This is,- for example, the case with the leaves of Begonia, and in a less 

 degree with those of the Elm. The whole radially symmetrical 

 plant body is here composed of dorsiventral and asymmetrical parts. 



