538 MICROSCOPIC FORMS OF VEGETABLE LIFE^THALLOPHYTES 



these are the special seat of the formation of the starch grains. If 

 coloured they are chromoplasts or chromatof)hores, the origin of the 

 various colouring matters of the cell ; those which give birth to the 

 chlorophyll corpuscles being distinguished by the special term chloro- 

 plasts. Minute bodies termed physodes, endowed with an amoeboid 

 motion, have been observed within the protoplasm filaments. In 

 some of the lower plants, at present exclusively in the green algae, 

 there are found within the chlorophyll corpuscles homogeneous 

 proteid substances known as pyrenoids ; they are often surrounded 

 by starch grains. 



The division of the nucleus may take place either directly, when 

 the process is known as fragmentation, or indirectly, when it is known 

 as mitosis or karyokinesis (see fig. 415). In the process of indirect 

 division, the protoplasm of which the nucleus is composed undergoes 

 a great variety of changes, in the course of which it assumes the 

 beautiful appearance known as the nuclear spindle, consisting of an 

 equatorial disc, the nuclear plate, and delicate spindle fibres which 

 converge towards the two poles of the spindle. Apparently con- 

 nected with the process of cell-division are the peculiar bodies 

 known as centrospheres, directing spheres, or attracting spheres, corre- 

 sponding to similar bodies found in animal cells, but at present 

 detected only in the lower forms of vegetable life. They form two 

 small homogeneous spheres lying near the nucleus, one on each side 

 of it, and imbedded in the cytoplasm. Each centrosphere has in its 

 centre a body termed the centrosome, composed of one or more small 

 granules. To follow out all the processes of karyokinesis requires 

 very high magnifying powers of the microscope, great skill in mani- 

 pulation, and the use of very delicate staining reagents. 



The older conception of the vegetable cell regarded it as a com- 

 pletely closed vesicle, the eiidoplasm of which is entirely shut off 

 from contact with that of the adjacent cells. Recent observations 

 require the modification of this conception. It has been shown that 

 in many cases the cell-wall is perforated by very minute orifices, 

 through w T hich excessively fine strings of protoplasm pass from one 

 cell-cavity to another (fig. 416). This continuity of protoplasm has 

 been observed in some seaweeds and other algae, in the endosperm 

 of the ovule, in the pulvinus or motile organ of the leaves of the 

 sensitive plant, and in many other instances, and is regarded by 

 some authorities as probably a universal phenomenon in living cells. 

 In the case of the sensitive plant it is undoubtedly connected with 

 the remarkable phenomenon of sensitiveness or irritability displayed 

 by the leaves. 



In the lowest forms of vegetation every single cell is not only 

 capable of living in a state of isolation from the rest, but even 

 normally does so ; and thus the plant may be said to be unicellular, 

 every cell having an independent * individuality.' There are others, 

 again, in which amorphous masses are made up by the aggregation 

 of cells, which, though quite capable of living independently, remain 

 attached to each other by the mutual fusion (so to speak) of their 

 gelatinous investments ; and there are others, moreover, in which a 

 definite adhesion exists between the cells, and in which regular 



