SCIENCE- G OSSIF. 



difficult. The foliage of one low plant would shade 

 that of its neighbours, and the individual with the 

 longest and most upright stem would receive the 

 greatest amount of light, and in consequence would 

 have a richer food supply and grow more vigor- 

 ously than its fellows. Such long-stemmed plants 

 would vanquish the shorter varieties when placed 

 in open competition with them, and a race with 

 long and rigid stems would tend to evolve. 

 ' Events of this nature most probably underlie the 

 history of our giant forest trees which raise their 

 heads of leaves high into the air and light. 



This enormous increase in the length of the 

 stem would necessarily be associated with consider- 

 able internal anatomical modification. Mechanical 

 strengthening tissue must be developed in far 

 greater amount to support the great weight of 

 leaves high above the earth ; the root system must 

 penetrate deeper and more firmly into the ground 

 to prevent the overthrow of the lofty sub-aerial 

 structure by every gust of wind, and this more 

 extensive root system would furnish a far broader 

 portal for the entrance of water into the plant. 

 The water-conducting channels must be elaborated 

 in response to this increased water supply and the 

 longer path which must be traversed. 



Moreover, the danger of evaporation from the 

 surface would be greater in this far journey from 

 root to leaf, and thicker layers of cork and bark 

 would be necessitated. All these modifications we 

 actually find in the tall trees which now and in the 

 past have peopled our earth. 



The manufacture of food is limited to the green 

 cells, and these are practically all gathered together 

 in the leaves. The food made in the leaves has to 

 be distributed all over the plant, even down to the 

 very tips of the roots. In the earlier plants we 

 find the food in a state of solution either passing- 

 down the same conducting channel, up which water 

 is moving, or taking a slow and laborious journey 

 from cell to cell of the general parenchymatous 

 tissue. In many ways such a course is an un- 

 satisfactory one, and very early in the course of 

 evolution we find a special conducting system 

 for dissolved food-stuffs developing in the stem. 

 Already in the higher mosses, such as the Poly- 

 trichaceae, we find such a food-carrying tissue, or 

 bast, appearing. In the higher plants, and possibly 

 even in the mosses, the bast consists chiefly of an 

 aggregate of sieve-tubes. Sieve-tubes are long 

 rows of cells, standing one above the other, in 

 which the separating cross-walls have not com- 

 pletely broken down, as we found in the vessels, 

 but are perforated by numerous tiny holes, giving 

 them the appearance of a sieve. Associated with 

 these sieve-tubes in the bast are also a considerable 

 number of parenchymatous cells. The bast, as a 

 rule, forms a mantle about the central water- 

 conducting column, but this arrangement is liable 

 to considerable alteration. 



Up to the present we have spoken almost entirely 



of the individual elements which are active in 

 carrying out the various functions of the plant, 

 and have only incidentally used the word " tissue." 

 Now we have to see that these elements, such as 

 vessels, sieve-tubes, cork cells, etc., are not scattered 

 anyhow through the plant without order or arrange- 

 ment, but are severally gathered together in definite 

 groups, which we may at once call tissues, and 

 that these are distributed in the plant with the 

 utmost symmetry and to the best advantage of the 

 organism. 



The primitive little-altered cells with which we 

 started our considerations form the ground- work of 

 the vegetable body in which the other tissues are 

 arranged. This aggregate of living almost un- 

 altered cells we have already called parenchyma. 

 The vessels or conducting-fibres that convey the 

 water are collected together, in company with a 

 few living cells, as the wood. The food-conducting 

 elements or sieve-tubes collectively compose the 

 bast, which, as we have seen, usually forms a mantle 

 outside the wood. 



Between the wood and the bast there are often 

 one or two layers of living actively dividing cells 

 which constantly, throughout the life of the plant, 

 add new elements to the bast on one side and the 

 wood upon the other. This active formative tissue 

 is called the cambium. 



The most central features in the anatomy of the 

 higher plants are the wood and the bast, which 

 together form, primitively, a single central column 

 running through the heart of the stem and the 

 root. Modern anatomists call this strand by the 

 Greek word for a column — namely, a stele. 



The outermost boundary of this column or stele 

 is very frequently marked by a peculiar and definite 

 layer of cells called the endodermis, the functions 

 of which are not properly understood as yet. 



The column of conducting-tissue sends branches 

 to each leaf, which carry water to, and food from, 

 these organs. As the leaves become more and 

 more numerous on the plant, branches are given off 

 from the central stele with increasing frequency- 

 A greater quantity of conducting-tissue is demanded, 

 and the more superficially this could be arranged 

 the easier would the entrance and exit of branches 

 be rendered. In answer to this demand we find 

 the originally solid column opening out into a 

 broad cylinder or ring enclosing only parenchyma 

 in the centre. This case of parenchyma is called 

 the pith. As the surface of the cylinder of con- 

 ducting-tissue broadens out still further, we find 

 the sieve-tubes and the vessels forming themselves 

 into little interrupted groups, still arranged in a 

 ring between which parenchyma gradually creeps, 

 forming the medullary rays running from the pith 

 like spokes of a wheel. Many other changes in the 

 form of the stele took place, as the development of 

 the plant world proceeded, but we have not space 

 to consider them in this paper. What I have 

 attempted to make clear to you in this note is that 



h 8 



