6 PART 1. THE MORPHOLOGY OF PLANTS. [ 1. 



Three main types of morphological constitution may be dis- 

 tinguished: 



1. The body is a thallus. 



2. The body consists of root and thalloid shoot (two dissimilar 

 members) . 



3. The body consists of root and leafy shoot (three dissimilar 

 members). 



These members frequently bear others of secondary morpho- 

 logical importance, such as hairs, prickles, and reproductive 

 organs. 



In highly organised plants the members can, as a rule, be 

 readily distinguished from each other ; but in some cases there 

 is difficulty in distinguishing between leaves and branches of the 

 stem, between leaves and hairs or prickles, between roots and 

 branches of the stem, etc. This difficulty is especially great in the 

 study of less highly organised forms (e.g. gametophytes of Algae, 

 Muscineae, and Pteridophyta), in which, whilst there is a certain 

 degree of morphological differentiation, it is insufficient to 

 obviously indicate the morphological nature of the members. In 

 such doubtful cases an investigation of the development, relations, 

 and structure of the member in question is the only method of 

 arriving at a conclusion as to its morphological nature. For the 

 principal members of the plant, stem, leaf, root, occupy certain 

 definite relative positions and present a general co-ordination of 

 structure, and are thus distinguishable from appendages such as 

 hairs, etc. Moreover, their relative positions serve to distinguish 

 them from each other; and though co-ordinate in structure, that 

 is, presenting a corresponding degree of complexity of internal 

 structure, yet they generally present distinguishing peculiarities 

 in the details. 



The morphological characteristics of the principal members are 

 briefly these : 



The shoot bears the true (spore-producing) reproductive organs. 

 It is frequently differentiated into stem and leaf. 



The stem is the axial member of the shoot, and bears the leaves. 



The leaf is the lateral member of the shoot ; it is borne upon 

 the stem, but usually differs from it in form and details of 

 structure, though stem and leaf are co-ordinate in structure. 



The ideas of stem and leaf are correlative, the one involving the other ; 

 nevertheless, in some cases (e.g. Kuscus, Cacti, etc.) in which leaves are, appar- 

 ently, not present, the axis is still termed a stem, because the shoot of such 



