90 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 21. 



the protoplasmic unit together with the wall (cell-wall} by which 

 it is invested, and to term the protoplasmic unit an energid. 

 The structure of the body or any part of it can only be accurately 

 described as cellular when it consists of one or more such cells, 

 that is, when it is either unicellular (e.g. Yeast, Haematococcus, 

 etc.) or multi cellular. The body of an unseptate plant (such as 

 the Phycomycetous Fungi and the Siphonaceous Algae), as also 

 a segment of the body of an incompletely septate plant (such as 

 Cladophora, Hydrodictyon, etc.), is not a single cell, but is an 

 aggregate of protoplasmic units (energids) enclosed within a 

 common wall. Such a body, or part of a body, may be con- 

 veniently distinguished as a coenocyfe, and the plants in which it 

 occurs may be said to have coenocytic structure. 



Even in typically cellular plants structures occur which are 

 coenocytic. Thus, in the early stages of its development in the 

 embryo-sac of a Phanerogam, the endosperm is generally unsep- 

 tate, consisting of a layer of protoplasm with many nuclei scattered 

 through it; it eventually becomes a cellular tissue by the delimit- 

 ation of the constituent energids by means of cell-walls. But even 

 when the cell-walls are formed, they do not always enclose single 

 energids ; in Corydalis cava, for instance, the net-work of cell-walls 

 encloses several energids in each mesh, so that the structure of the 

 endosperm is at first coenocytic; eventually, however, the nuclei in 

 each ccenocyte fuse together until only one remains, and in this 

 way the transition from ccenocytic to cellular structure is effected. 

 Again, a " laticiferous cell" of a Euphorbia (and other Phanero- 

 gams) is essentially a ccenocyte like the body of a Vaucheria or a 

 Botrydium. 



On the other hand, there is such a thing as a multinucleate 

 cell. It has been observed, for instance, in old internodal cells of 

 Chara, and in old parenchymatous cells of Lycopodium and of 

 various Phanerogams (e.g. Tradescantia, Taraxacum, Cereus, Sola- 

 num, etc.) that, from being uninucleate, they become multinucleate 

 by the direct division or fragmentation of the nucleus (see p. 96). 



The distinction between a ccenocyte and a multinucleate cell 

 would appear to be this : that the former is either multinucleate 

 from the first or becomes so at a very early stage in its develop- 

 ment, whilst the latter becomes multinucleate at a quite late 

 period ; and further, that in the ccenocyte the nuclei multiply by 

 indirect division (see p. 97), whereas in the multinucleate cell 

 they multiply by direct division or fragmentation. 



