476 PHYSIOLOGY. 



tubular bodies, which, in their various conditions, form what are 

 called the ducts and vessels of plants. These constitute the Vascu- 

 lar Tissues. 



Vascular tissues. What are called the vessels of plants are really 

 compound elementary organs ; but it is not requisite to enter into more 

 minute distinctions here, since the phenomena of fusion of cells into 

 such compound organs are not very varied in plants, and in all cases the 

 composition of the structure from a number of distinct cells is very 

 evident. 



The tissues, simple and compound, enter into the composition of the 

 Organs of Vegetation and Eeproduction of Plants upon a certain general 

 plan for any particular kind of organ, but under specially modified 

 arrangements, referable to a progressive series of types, in the several 

 large Classes of the Vegetable Kingdom. 



Sect. 2. THE CELL. 



Form. The shape and sizes of the cells of plants are determined 

 by causes of two kinds, namely : their own laws of growth, which 

 are inborn and hereditary ; and the favourable or obstructive influ- 

 ences which bear upon their development in each particular case. 

 As a general statement, it may be said that the primary form of the 

 Vegetable Cell is that of a sphere, and that deviations from that 

 type are more or less attributable to secondary influences, arising 

 from the connexion of cells in coherent groups. 



The spherical form is usually found in cells developed freely, i. e . not 

 arising from mere subdivision of a preexisting cell. Thus we find em- 

 bryonary cells and endosperm-cells in the embryo-sac of Phanerogamia, the 

 spores of some Cryptogamia, together with many of the lower plants com- 

 posed of one or few cells only, such as those of 'growing Yeast (fig. 587, 

 p. 552), &c., presenting the spherical as the original form. But by far the 

 most frequently occurring spherical cells, such as many poll en-grains, spores, 

 those in the pith of young shoots of Dicotyledons, of the pulp of fruit, &c., 

 assume this form subsequently to the earliest stage of development, being 

 placed in circumstances which allow them to expand freely according to 

 their natural tendency. 



The above general statement is subject to certain important exceptions, 

 in which deviation from the typical form exists without any interference 

 with the development of the cell according to its own laws ; these are 

 met with principally in the lower cellular plants^ especially the Unicel- 

 lular Algse, in which we find single free cells assuming the most varied 

 but specifically determinate forms. 



Examples of this are offered not only by the Desmidiete, but by the 

 more unequivocally vegetable Vaucheriee, Botrydium (fig. 503, E), and 

 others. 



The interfering influences above referred to are of two principal kinds, 

 namely : special directions assumed in the development, in obedience to 



