THE STRUCTURE AND FUNCTION OF PLANT-ORGANS 39 



that aerial organs are commonly developed by fungi and other plants which 

 can grow in darkness. 



The study of comparative morphology reveals all stages of differentia- 

 tion in form and function ; thus, in the development of the plant from the 

 ovum or spore, progressive differentiation takes place, the extent of which 

 varies very much in different plants. The term ' thallus ' may be used to 

 represent the least amount of differentiation possible in a multicellular 

 plant, but the distinction is not always definite l , for every phanerogamic 

 embryo passes through a thalloid condition. A thalloid form can be 

 differentiated into shoot and root, and it is only in order to indicate the 

 simplicity of the latter in thalloid forms, such as algae, fungi, and hepa- 

 ticae, that we often speak of rhizoids instead of roots. 



Even non-septate plants may exhibit marked morphological differen- 

 tiation ; for example, some species of Caulerpa simulate the form of the 

 vegetative organs of Phanerogams. In plants of considerable size, how- 

 ever, the multicellular condition is very important, and indeed essential to 

 give sufficient strength and rigidity. A large non-septate plant is very 

 liable to be fatally affected by a purely local injury. For these and other 

 reasons, the non- septate condition is impossible for large plants which may 

 be in giant forms as much as 140 metres high, but possible and indeed 

 advantageous for the smaller forms of which the diameter may not 

 exceed o-ooi mm. 



The cell- membrane secreted by the protoplasm serves primarily to give 

 rigidity to the plant as a whole, and forms in trees the skeletal framework 

 within which the softer parts are protected from pressure and strain. This 

 solid and permanent envelope, by which the protoplast is enclosed, circum- 

 scribes the movement of which the latter is capable, and renders impossible 

 any amoeboid changes of form on the outer surface which is closely 

 adpressed to the cell-wall. The protoplast can increase the size of its 

 dwelling-place, and induce permanent alterations of shape, only so long as 

 it can cause a growth of the cell-wall to take place. 



Peculiarities of form, of the mechanism of movement, and of the mode 

 in which nutriment is obtained, arise owing to the permanent fixation to 

 the substratum. The organisms compelled to live fixed to the ground 

 must necessarily differ in many respects from those which are capable 

 of translatory movement. For this reason it is all the more important 

 that full attention should be paid to those plants which can move from 

 place to place, in order that we may estimate more correctly the different 

 phenomena exhibited to us by plants in general, and may also be able to 

 establish the relations between plants and animals. 



In the tissues of the higher plants a more or less marked differentiation 



' Goebel, Vergleichende Entwickelungsgeschichte d. Pflanzenorgane, 1883, pp. 127, 131. 



