DIV. i MORPHOLOGY 73 



I. Vegetative Organs 



The highest segmentation attained by the vegetative organs of 

 plants is that into ROOT, STEM, and LEAVES. Stem and leaves are 

 classed together as the SHOOT. A plant body composed of shoot and 

 root is termed a CORMUS, and plants so constructed CORMOPHYTES. 

 The fern-like plants, or Pteridophyta > and the more highly-segmented 

 seed plants derived from them are cormophytes. 



The cormophytes arose phylogenetically from more simply 

 organised plants in which the plant body had not attained such a 

 profound segmentation ; in which roots were wanting, while leaf-like 

 branches though not true leaves were present. Such structures, as 

 well as quite simple and unsegmented plant bodies, are included 

 under the term THALLUS, and such plants may be contrasted with 

 the cormophytes as thalloid. The Algae, Fungi, Lichens, and all 

 Bryophyta have thalli. 



The thalloid plants must not be confused with the Thallophyta. All thalloid 

 plants possess a thallus, but they are not all Thallophyta. Under this name 

 systematic botany includes only the Algae, Fungi, and Lichens. 



A. THE THALLUS ( 50 ) 



(a) Algae, Fungi, Lichens. 1. Simplest Forms. The only forms 

 that are quite unsegmented externally are a number of microscopically 

 small unicellular or multicellular plants. The simplest form that can 

 be assumed by an organism is that of the sphere. 



For example, such spherical cells are shown by some Algae that form a green 

 coating on damp walls (Fig. 35), and by many Bacteria (Fig. 80 b). The latter 

 include by far the smallest known organisms. 



2. Increase of Surface. Of all geometric figures the sphere has 

 the smallest surface for the same volume, and this surface bears a 

 smaller ratio to the volume the greater the latter is. Deviations 

 from the spherical form are thus connected with a relative increase of 

 the surface. In particular, as the volume of the body increases the 

 surface area is in this way increased relatively to the volume. 

 Cylindrical, rod-shaped, filamentous, ribbon-shaped, and discoid forms 

 thus occur, and ultimately bodies segmented by reason of their external 

 projections. The free surface of the body is of great importance to 

 the plant for the absorption of the gaseous and liquid substances 

 necessary for its nutrition and derived from the environment. 



Even when spherical the cells of Bacteria on account of their minute size have 

 an extraordinarily large free surface as compared with cells of higher organisms. 



The unicellular individuals of the beer Yeast (cf. Fig. 20) are ellipsoidal in 

 shape, while the cells of many Algae, such as species of Diatoms (Fig. 79), are 

 discoid or cylindrical. This group of Algae exhibits spindle, canoe, helmet, and 



